Syrian troops and militia loyal to President Bashar al-Assad captured and then shot dead 27 young men in northern villages and two foreign journalists were killed in shelling of the besieged city of Homs, activists said on Wednesday.

The two Western journalists were killed on Wednesday when shells hit the house they were staying in, activists and witnesses said. They were named as Marie Colvin, an American working for Britain's Sunday Times newspaper, and French photographer Remi Ochlik.

A witness told Reuters by phone that shells hit the house where the journalists were staying and a rocket hit them as they were escaping.

Violence continued to spread. Several YouTube videos taken by local activists in Idlib, which could not be independently confirmed, showed bodies of young men with bullet wounds and hands tied lying dead in streets.

The men, all civilians, were mostly shot in the head or chest on Tuesday in their homes or in streets in the villages of Idita, Iblin and Balshon in Idlib province near the border with Turkey, the Syrian Network for Human Rights said.

Military forces chased civilians in these villages, arrested them and killed them without hesitation. They concentrated on male youths and whoever did not manage to escape was to be killed, the organisation said in a statement.

Responsibility for this massacre lies with the general commander of the military and armed forces, Bashar al-Assad, the statement said, adding that only one youth survived the shootings.

One video shows the body of three youths, one visibly shot in the chest, on the floor of a house in Balshon.

This is martyr Hassan Abdel Qadi al-Saeed, his brother Hussein and (their relative) Bashir Mohammad al-Saeed. They were liquidated by Assad's forces in the February 21 massacre, a voice of a man showing the bodies says, with the sound of women wailing in the background.

The raids came as the United States appeared to open the door to eventually arming the Syrian opposition, saying that if a political solution to the crisis was impossible it might have to consider other options.

The comments, made by officials at both the White House and the U.S. State Department on Tuesday, marked a shift in emphasis by Washington, which so far has stressed a policy of not arming the opposition and has said little about alternatives.

We still believe that a political solution is what's needed in Syria, White House spokesman Jay Carney said.

We don't want to take actions that would contribute to the further militarisation of Syria, because that could take the country down a dangerous path. But we don't rule out additional measures.

Asked if the United States was shifting its stance on arming the rebels, U.S. State Department spokeswoman Victoria Nuland said Washington did not want to see the violence increase and was concentrating on political efforts to halt the bloodshed.

That said ... if we can't get Assad to yield to the pressure that we are all bringing to bear, we may have to consider additional measures, she said, declining to elaborate.

U.S. Secretary of State Hillary Clinton will meet representatives of some 70 countries and organisations in Tunis on Friday for the first Friends of Syria meeting to coordinate the international community's next steps in response to the nearly year-long uprising against Assad.

The United States and its allies hope the Tunis conference will allow them to begin drawing up a plan for Syria after Russia and China vetoed a Western-backed Arab League peace plan at the U.N. Security Council.

With both Russia and Iran firmly backing Assad's government, political analysts say tacit U.S. support for arming rebel fighters could be risky given Syria's complex ethnic and religious make-up and strategically important position.

ARMY PREVENTS AID SUPPLIES

In Syria on Tuesday, activists said Assad's forces killed more than 60 people in attacks on villages and an artillery barrage on the restive city of Homs. The Red Cross called for daily ceasefires to allow in urgently needed aid.

A Syrian opposition figure who managed to get into Homs appealed for international help.

The sound of bombardment and sniper fire are echoing across the city, Moulham al-Jundi, a member of the Syrian National Council, told Reuters from Homs.

The army prevents first aid or medical supplies from going in and electricity is cut off 15 hours a day. There has been no mobile phone service for three weeks, said Jundi, who lives in exile in Saudi Arabia and was smuggled into Homs.

Civilians need safe zones and a way has to be found to ensure that medicine and basic supplies reach Homs. There are no hospitals, no schools, no work, no government departments open and most shops are shut.

A delegation from the Syrian National Council, the main Syrian opposition grouping, is due to meet Red Cross officials in Geneva on Wednesday.

Activists' accounts of the violence could not be independently verified. The government bars most foreign journalists from Syria.

Official media said government forces were targeting armed terrorist groups who have been terrifying citizens and attacking security forces and robbing public and private property.

(Editing by Giles Elgood)