The United States has sent a letter to Iran regarding its threatened closure of the strategic Strait of Hormuz, Iran’s foreign ministry spokesman said on Sunday, without revealing the letter's contents.

"The U.S. ambassador to the U.N., Susan Rice, sent a letter to Mohammad Khazaie, Iran's U.N. representative, which was conveyed by the Swiss ambassador, and finally Iraqi President Jalal Talabani delivered its contents to officials" in Iran, the official IRNA news agency quoted Ramin Mehmanparast as saying.

The U.S. government is concerned that Israel is preparing to take military action against Iran, and has stepped up contingency planning to safeguard U.S. facilities in the region, The Wall Street Journal reported late Friday.

The newspaper said President Barack Obama, Defense Secretary Leon Panetta and other top officials have delivered a series of private messages to Israeli leaders, warning about the dire consequences of a strike.

U.S. President Barack Obama spoke by telephone Friday with Turkey's Prime Minister Recep Tayyip Erdogan about the situation in Syria and Iran's nuclear program, the White House said.

"They agreed that the U.S. and Turkey should continue to support the legitimate demands for democracy for the Syrian people and condemned the brutal action of the Assad regime," it said in a statement.

The United States believes Iran is supplying munitions to aid Syria's bloody protest crackdown in an initiative spearheaded by Tehran's revolutionary guard supremo, senior U.S. officials told Agence France Presse Friday.

Qasem Soleimani, head of the Iranian Revolutionary Guards Corps elite Quds force, was in the Syrian capital this month, one official said, in what Washington sees as the most concrete sign yet that Iranian aid to Syria includes military hardware.

The United States said Friday it has raised concerns with Russia and Cyprus over a Russian ship suspected of carrying munitions to Syria.

State Department spokeswoman Victoria Nuland said "we have raised our concerns about this, both with Russia and with Cyprus, which was the last port of call for the ship."

The United States has used a secret channel to warn Iran's leaders against closing the strategic Strait of Hormuz, saying that doing so would provoke a U.S. response, the New York Times reported.

Iran has threatened to close the narrow and strategic waterway -- a chokepoint for one fifth of the world's traded oil -- in the event of a military strike or the severe tightening of international sanctions.

The United States plans to withdraw about 7,000 U.S. troops of the 81,000 troops based in Europe, U.S. Defense Secretary Leon Panetta said Thursday.

In an interview with the Armed Forces Press Service, Panetta said two brigade combat teams, or roughly 7,000 U.S. troops, would be withdrawn from Europe, but rotational units would still maintain strong military presence in the region.

Four Americans were being held by security forces after they were stopped for "suspicious" activity in the center of Baghdad on Thursday, the governor of Baghdad province told Agence France Presse.

The two men and two women were driving a BMW with local license plates, rather than diplomatic registration, and were all wearing flak jackets and armed with pistols and automatic weapons, said Salah Abdulrazzaq, who described the group as "gunmen."

Afghan President Hamid Karzai Thursday strongly condemned a video appearing to show U.S. soldiers "desecrating" the corpses of slain insurgents by urinating on them.

"The government of Afghanistan is deeply disturbed by a video that shows American soldiers desecrating dead bodies of three Afghans," said a statement from the president's office.

Iran's President Mahmoud Ahmadinejad slammed capitalism as bankrupt and called for a new world order Wednesday on a visit to Cuba, steering clear of the controversy over his country's nuclear program.

The Iranian leader arrived in Cuba for talks with his counterpart Raul Castro as the Islamic republic blamed Israel and the United States for the killing of a nuclear scientist in a Tehran car bombing.