The USS Nimitz aircraft carrier is moving westward toward the Red Sea, although it has not yet received orders to support a potential United States strike on Syria, ABC News reported on Monday.

The Nimitz carrier strike group, which includes a guided missile cruiser and four destroyers, was kept in the Indian Ocean for a "prudent responsible decision", an official told the television channel.

A US defense official confirmed to AFP that the carrier was in the region but declined to give a specific location.

"Overlap of navy vessels is routine as responsibilities are passed and the number of ships in a given area of responsibility vary," the official said. "However, we are preserving options by keeping some additional ships in the region to include the USS Nimitz aircraft carrier."

The nuclear-powered carrier was set to head back to its home port in Everett, Washington, after a months-long deployment to the Arabian Sea when it reportedly received orders to stay in the area.

It has not been assigned a specific mission and defense officials described the move as "prudent positioning", according to ABC.

Five US destroyers are now positioned in eastern Mediterranean waters, up from the usual three that normally focus on countering Iranian ballistic missile threats to Europe.

The destroyers the USS Stout, Mahan, Ramage, Barry and Graveley - are ready to fire cruise missiles if US President Barack Obama gives the order.

The USS San Antonio, a navy amphibious ship with several helicopters and hundreds of Marines on board, has also been ordered to be stationed in the eastern Mediterranean, but a defense official said it has "received no specific tasking".

No amphibious landing is in the works, however, as Obama has ruled out any "boots on the ground" if the US takes military action against the Syrian government.

Obama's decision to ask Congress to authorize military action on Saturday lifted the threat of immediate strikes on President Bashar al-Assad's government.

In a separate development, the US decision to postpone possible strikes on Syria has done little to allay fears in northern Israel, where lines at post offices were backing up as civilians collected gas masks.

Talk of imminent US military action against Damascus had sparked widespread panic in Israel.

Fearing the fallout from any attack on Syria could spill across their northern border and drag the Jewish state into the conflict, Israelis last week rushed to replace their old gas masks.

But lines continued to back up for hours as many Israelis, particularly in the north near the borders with Syria and Lebanon, insisted on collecting protective gear that has been distributed at post offices across the country.

Local media reported on Monday that some 40 percent of the population still did not have gas masks.