The Japanese government is not convinced by the US claim that Iran is to be blamed for the attacks on two tankers near the Strait of Hormuz on Thursday, government sources said Sunday.

The Japanese government has been requesting the US for concrete evidence to back its assertion that Iran attacked tankers. The request came after US Secretary of State Mike Pompeo gave a statement hours after the attacks blaming Iran but without offering proof. The Department of Defense later released a video allegedly showing an Iranian patrol boat removing an unexploded mine attached to the side of the Japanese-operated tanker Kokuka Courageous.

But Japanese government officials remain unconvinced, the sources said. "The US explanation has not helped us go beyond speculation," said one senior government official.

Japan has been seeking more concrete evidence through various channels, including Foreign Minister Taro Kono who is likely to have made the request during a call with his counterpart on Friday, the sources said.

Pompeo said in a press conference Thursday that the United States' assessment was based on their "intelligence, the weapons used, the level of expertise needed to execute the operation, recent similar Iranian attacks on shipping, and the fact that no proxy group operating in the area has the resources and proficiency to act with such a high degree of sophistication."

A source close to Prime Minister Shinzo Abe said, "These are not definite proof that it's Iran."

"Even if it's the United States that makes the assertion, we cannot simply say we believe it," he said.

While the Japanese Prime Minister was visiting Iran after 4 decades and many expected even more reduction of the tensions in the region due his visit, in another suspicious and provocative move two large tankers were hit by explosions in the Sea of Oman on Thursday, a move that can intensify the tensions more than before.

Following the attack, US Secretary of State Mike Pompeo hastily accused Iran of carrying out the attacks without providing any evidence to back up his accusation.