The US is not giving the public a heads up on when it will strike IS in Syria

The United States is ready to launch air strikes on Islamic State targets in Syria but does not want to telegraph when these will occur, President Barack Obama's National Security Adviser Susan Rice said on Friday.

Her comments underline mounting questions over when the United States will expand its air campaign against Islamic State into Syria from neighboring Iraq after Obama laid out guidelines for strikes aimed at denying Islamic State militants a safe haven in either country.

"I don't think it would be appropriate or wise for me to telegraph from the podium exactly when that will occur, and what steps may need to be taken before that is to occur," Rice told reporters at the White House.

"I'm not going to give you any precision or prediction on when that might occur," she said.

White House spokesman Josh Earnest said each air strike would not require approval by Obama. "Contrary to some published reports, he's not going to be in a position where he's approving or disapproving individual air strikes in Syria," he said.

But Earnest declined to say whether Obama would need to approve the first strike in Syria, saying only that Obama and his team were reviewing plans laid out by the Pentagon.

Earnest also said that the United States was taking precautions to limit damage or injury to civilians in Iraq and Syria.

Meanwhile, the United States has discussed the threat posed by Islamic State militants with envoys from its traditional foe Iran on the sidelines of nuclear talks, the State Department revealed Friday.

The department's director of press relations Jeff Rathke said the issue was separate from the nuclear agenda, "but that discussion on this threat did arise on the margins of the meeting."

US Congress approves arming Syrian rebels

The US Congress gave final approval on Thursday to President Barack Obama's plan for training and arming moderate Syrian rebels to battle Islamic State, a major part of his military campaign to "degrade and destroy" the militant group.

The Senate voted 78-22, in a rare bipartisan show of support for one of Obama's high-profile initiatives. With the House of Representatives approving the legislation on Wednesday, the measure now goes to Obama to sign into law.

Ten Senate Democrats and 12 Republicans voted no.

Obama thanked Congress for the speed in which it acted to back the plan, which he announced on Sept. 10, and said the strong bipartisan support showed Americans were united in the fight against Islamic State.