WASHINGTON - US Defense officials said on Wednesday that they believe tensions between the United States and Iran have cooled down recently, despite a bomb attack killed an Iranian nuclear site staff and his driver in northern Tehran on Wednesday.

Pentagon Press Secretary George Little made the statement during a press briefing, saying the United States has been seeking "to lower the temperature on tensions with Iran, and we think that things have calmed down a bit in recent days."

However, the United States still maintain sizable naval forces near Iran.

John Kirby, another Pentagon spokesman, justified the presence of two US carrier groups in the US Central Command area of operations as "prudent force posture requirements set by the combatant commander," which is nothing out of the ordinary.

Kirby said during the same briefing that the two carrier groups in the region continue the nearly constant US naval deployments to the region since World War II, and is not tied to recent strains with Iran.

"I don't want to leave anybody with the impression that ... we' re somehow 'zorching' two carriers over there because we're concerned about what happened ...today in Iran," he said, underlining the fact that the two carriers in the region is not an indication of any specific trouble with respect to Iran. He said neither carrier group is inside the Strait of Hormuz.

Iranian media reports indicated Mostafa Ahmadi-Roshan, deputy chief of the commercial section of Iran's Natanz nuclear enrichment site, was killed in a bomb attack Wednesday.

A number of Iranian nuclear scientists have been assassinated in recent years. Iran's First Vice President Mohammad-Reza Rahimi said Wednesday that Israeli agents were the perpetrators of Ahmadi- Roshan's assassination, local media reported.