ANKARA - The United States and Britain have suspended all non-lethal aid to the opposition in northern Syria after rebels seized key bases and warehouses belonging to the Western-backed Free Syrian Army, a US official said Wednesday.

“We have seen reports that Islamic Front forces have seized the Atmeh headquarters and warehouses belonging to the Supreme Military Council and we are obviously concerned,” T.J. Grubisha, spokesman for the US embassy in Ankara, told AFP.  “Because of the current situation, the United States has suspended deliveries of non-lethal assistance into northern Syria,” he said.

“We have no plans to deliver any equipment while the situation remains unclear,” a spokesman for the British embassy in Ankara told AFP.

The Islamic Front, the largest Islamist rebel force in Syria, seized depots belonging to the FSA near the Bab el-Hawa border crossing with Turkey on Saturday. The group on Tuesday also took control of the crossing itself, the Syrian Observatory for Human rights, a monitoring group, said. Non-lethal aid provided to the rebels by Washington has included armoured vehicles, night vision goggles and advanced communications equipment. Grubisha said the US decision to suspend such aid would not impact on humanitarian assistance which was coordinated by international and non-governmental organisations including the United Nations. “We are gathering the facts and consulting with friends of the Syrian opposition on next steps in support of the Syrian people,” Grubisha said.

“We are working with General Idriss and SMC staff to inventory the status of US equipment and supplies provided to the SMC,” he added, referring to the Supreme Military Council of the FSA headed by Selim Idriss. The Islamic Front was formed last month when six groups merged and pledged to work towards forming an Islamic state. It has rejected the authority of the FSA. Turkey on Tuesday shut its side of the border in Hatay province after the Islamic Front seized the Bab al-Hawa post, customs officials said in a statement.

“The crossing into Syria via Cilvegozu has been closed ... due to the clashes between rebel groups in Syria,” the statement said. But a foreign ministry official, contacted by AFP, said the measure was temporary and would not affect refugees entering Turkey.

Meanwhile, Russian FM Sergei Lavrov urged all “responsible countries” to act to ensure a Syrian peace conference achieves positive results, in remarks made in Iran on Wednesday.

Iran and Russia back President Bashar al-Assad’s regime in the Syrian conflict, which is estimated to have killed almost 126,000 people over nearly three years.

Lavrov arrived in Tehran late Tuesday for talks officials said would focus on the Syria peace conference dubbed Geneva 2 set for January 22, as well as bilateral ties and Iran’s landmark nuclear deal with world powers.

He was to meet President Hassan Rouhani later Wednesday, Iran’s foreign ministry said.

“All responsible countries must do something so that Geneva 2 achieves a positive result,” Lavrov said at a joint news conference with his counterpart Mohammad Javad Zarif.

“Those who are against (such a result) show a lack of commitment faced with the demands of the international community.”

Lavrov repeatedly called Iran a “key player” that could help resolve the Syrian conflict and should be invited to Geneva.

The Syrian regime and the opposition have stated their willingness to attend Geneva 2, but they differ on how the transition of power and on officials from Iran and Saudi Arabia, which backs the rebels, attending the talks.

Damascus says Assad will stay on as president and lead any transition agreed at the conference.

But the opposition and rebels reject Iranian attendance at the conference and insist Assad has no role in any transition.

Zarif repeated that Iran was ready to go to Geneva but “without precondition”, and said the conflict should be solved by the Syrian people.

On Russian arms sales to Iran, Zarif was asked whether Tehran would consider alternatives to the S-300 missile system it has ordered.

“We still insist on the implementation of the previous agreements” with Moscow, he replied, adding that “a mutually acceptable solution” would be found.

In 2007, Russia signed a contract to supply Iran with five of the advanced S-300 ground-to-air systems - which can take out aircraft or guided missiles - at a cost of $800 million.

In 2010, Russia’s then president Dmitry Medvedev cancelled the contract after coming under US and Israeli pressure.

Tehran then lodged a lawsuit at an international court in Geneva against Russia’s arms export agency.

Lavrov’s two-day visit comes after Iran and the P5+1 — the United States, Britain, France, China, Russia plus Germany — agreed an interim nuclear deal on November 24.

Under the Geneva accord, Iran agreed to freeze or curb some of its controversial nuclear activities in return for limited relief from crippling Western sanctions.

Zarif said Iran and Russia were closely cooperating to “form the final agreement which would be the difficult phase of future negotiations with P5+1”.

He said construction of civil nuclear facilities was discussed, adding that “in some cases we are very close to an agreement and its implementation”.

Iran’s sole nuclear power plant at Bushehr, built by Russia, entered service in 2011 and produces 1,000 megawatts.

Tehran aims to produce 20,000 megawatts of electricity from nuclear power, which would require 20 similar reactors.

Lavrov said Moscow was interested to continue working with Iran on expanding “its peaceful nuclear programme”.

“I emphasise that no UN resolution has prohibited such a reactor” similar to Bushehr, he said.

Western powers and Israel suspect Iran’s nuclear programme masks a covert weapons drive, a charge Tehran denies, saying it is entirely peaceful.

Turkey, which is fiercely opposed to the regime of President Bashar al-Assad, has taken in about 600,000 refugees from the Syrian conflict and has become a base for Syrian rebels and army defectors who form the very core of the FSA.

The FSA was the first organised rebel military entity to emerge after peaceful demonstrations against the Assad regime in 2011 degenerated into an armed uprising.

Since then, numerous other groups have emerged, many of them Islamist, which are operating with a high degree of autonomy from the FSA if not totally independently.

Syria’s civil war, which has killed more than 126,000 people and displaced millions, has become more complicated as jihadist and other rebel groups battle not only Assad but, increasingly, each other.

The United States has not committed to providing any weaponry for Syrian rebels out of fear that it may end up in the hands of extremists.

Meanwhile, three of the six countries not covered by the Chemical Weapons Convention are close to joining the agreement, the head of the world’s chemical watchdog said Wednesday.

Speaking in Oslo the day after the Organisation for the Prohibition of Chemical Weapons (OPCW) formally received the Nobel Peace Prize, director general Ahmet Uzumcu said Angola, Myanmar and South Sudan “are very close.”

“The three others have other concerns” which could be related to “regional reasons”, he added during a meeting with Norwegian lawmakers broadcast by the NRK public service network.

The Chemical Weapons Convention - which outlaws the production, stockpiling, and use of chemical weapons - entered into force in 1997 and has 190 member countries including Syria, the latest nation to join in October this year.

Only six countries remain outside: Israel and Myanmar have signed the pact but not ratified it, while Angola, Egypt, North Korea and South Sudan have failed to do either.

During Tuesday’s Nobel ceremony, both Uzumcu and the chairman of the Norwegian Nobel Committee, Thorbjoern Jagland, urged the remaining countries to join the convention.