The Japanese government has been requesting the United States for concrete evidence to back its assertion that Iran is to blame for the attacks on two tankers near the Strait of Hormuz on Thursday, government sources said Sunday.

The request came after U.S. 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 U.S. 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.

If having expertise sophisticated enough to conduct the attack could be a reason to conclude that the attacker was Iran, "That would apply to the United States and Israel as well," said a source at the Foreign Ministry.

The attacks occurred around the time Abe was meeting with Iranian Supreme Leader Ayatollah Ali Khamenei in Tehran.

"The attacks have severely affected the prime minister's reputation as he was trying to be a mediator between the United States and Iran," said the source close to the premier. "It is a serious concern, and making mistakes when determining facts is impermissible."

The Japanese government has refrained so far from commenting on who is responsible for the attacks.