A French photojournalist and a prominent American war correspondent working for a British newspaper have been killed as Syrian forces shelled the opposition stronghold of Homs.

President Bashar Assad's regime also escalated attacks on rebel bases elsewhere, with helicopter gunships hitting areas in the northwest, activists said.

The latest deaths further increased international pressure on Assad, who appears intent on widening his military crackdowns despite the risk of pushing Syria into full-scale civil war.

"This tragic incident is another example of the shameless brutality of the Assad regime," US State Department spokeswoman Victoria Nuland said of the journalists killed.

The Obama administration opened the door slightly on Tuesday to international military assistance for Syria's rebels, with officials saying new tactics may have to be explored if Assad continues to defy pressure to halt a brutal crackdown on dissenters that has raged for 11 months and killed thousands.

The White House and State Department said they still hope for a political solution. But faced with the daily onslaught by the Assad regime against Syrian civilians, officials dropped the administration's previous strident opposition to arming anti-regime forces. It remained unclear though what, if any, role the US might play in providing such aid.

France was outraged over the journalists' deaths. "That's enough now, the regime must go," said French President Nicolas Sarkozy.

French spokeswoman Valerie Pecresse identified those killed as French photojournalist Remi Ochlik, 28, and American reporter Marie Colvin, who was working for the Sunday Times.

Syrian activists said at least two other Western journalists were wounded in the shelling, which claimed at least 13 lives. A Homs-based activist, Omar Shaker, said the journalists were killed when several rockets hit a garden of a house used by activists and journalists in the besieged neighbourhood of Baba Amr, which has come under weeks of heavy bombardment by forces from Assad's regime.

Ms Colvin, from Oyster Bay, New York, was in her 50s and a veteran foreign correspondent for the Sunday Times for the past two decades. She was instantly recognisable for an eye patch worn after being injured covering conflicts in Sri Lanka in 2001. Ms Colvin said she would not "hang up my flak jacket" even after the eye injury.