Pakistan has told the Obama administration that it is sending an additional six army brigades to join a major government offensive against Taliban forces in the northwestern part of the country, and it has pledged to hold territory where extremist forces are dislodged, Pakistani and U.S. officials said yesterday. Pakistani President Asif Ali Zardari said domestic support for the offensive, combined with U.S. assistance, would allow the effort to succeed where two previous military drives into the Swat Valley and surrounding territory failed. In an interview yesterday with Washington Post reporters and editors, Zardari did not confirm the movement of the brigades, some of which were said by others, on the condition of anonymity, to be moving from Punjab province and the country's border with India. But the United States and Pakistan, Zardari said, had "gotten to an understanding where we will be supported in all fields." During a White House meeting with Zardari on Wednesday, President Obama said, "I think we agreed that Pakistan needs more help." Congress has questioned whether Pakistan will effectively use the billions in economic and military assistance Obama has requested. To aid in the attacks, the Pentagon is speeding spare parts, ammunition and other equipment for Pakistan's fleet of aging Cobra attack helicopters. An earlier Pakistani request for more Cobras, to add to its existing fleet of two to three dozen, has been slowed, officials said, by the fact that the helicopters are no longer in production and aircraft must be located and refurbished. Zardari and Afghan President Hamid Karzai traveled to Washington this week for separate meetings with Obama and for a tripartite summit that the administration hoped would improve relations between the South Asian neighbors. Their mutual suspicion has undercut their overlapping fight against extremists. Administration officials pronounced the White House meetings -- and separate sessions among senior Pakistani, Afghan and U.S. intelligence, diplomatic, agriculture and other Cabinet-level officials -- a success. But they cautioned that they would await follow-through on promises made by both leaders. "The tension has gone. Reality has set in," Karzai told reporters yesterday. Volubly upbeat during public appearances, Karzai repeatedly referred to Zardari as his "brother." Administration officials said the two governments agreed to strengthen cooperation on a number of fronts, including trade and transit, as well as monitoring the traffic of Taliban fighters across their joint frontier. They said they would add two border coordination centers to the one currently in existence. U.S. military and intelligence officials worry that Taliban forces pushed out of Afghanistan by reinforced U.S. troops this summer will flow unimpeded into Pakistan, as they did during U.S. operations in Afghanistan in 2001. The Pakistanis, a senior Obama administration official said, need to "get ready for the influx . . . into western Pakistan, particularly Baluchistan" province. In the interview, Zardari described the $15 billion sent to Pakistan over the past decade in counterterrorism reimbursements and direct assistance as only a small fraction of the funds provided recently to failing U.S. financial institutions, adding that "the situation in Pakistan is much more important." The Pakistani economy needs to be boosted, he said, with "some form of a permanent stimulus." Zardari said that no one in the U.S. government had asked him for more information about the location and security of Pakistan's nuclear arsenal. "By and large, we've always had a relationship that is quite comfortable in the sense that people who need to know, know." Asked if any American officials knew "everything" about Pakistan's nuclear arsenal, he responded: "Every country has a right to their own sovereignty. We don't ask you personal questions, and you don't ask us." He said he continued to request that Pakistan be given its own fleet of U.S. aerial drones to attack Taliban and al-Qaeda sanctuaries. "Maybe some people here would not like to go to that direction," he said, "but . . . I keep asking." Zardari said his first meeting with Obama since the U.S. inauguration was "a very good start."