BEIRUT - At least 40 people were killed across Syria on Friday, including 26 pro-government militiamen, the opposition reported.

The militiamen, known as Shabihaa, were killed in the northern town of Daraat Azza, which has been a target of shelling attacks by government troops in recent weeks, said the opposition Syrian Observatory for Human Rights.

State television said that "an armed group" had kidnapped and killed 25 people in Daraat Azza, in the northern province of Aleppo.

Meanwhile, 24 civilians were killed as Syrian troops shelled rebel strongholds in the central province of Homs, Daraa in the south and Idlib near the border with Turkey, the organisation said.

Red Cross aid workers returned on Friday to the capital, Damascus, after failing to gain access to Homs, which military troops targeted in shelling attacks, according to opposition activists.

Earlier this week, the International Committee of the Red Cross (ICRC) said it would assist civilians caught in the fighting between Syrian forces and rebels in Homs, which has been the target of bombardment by forces loyal to President Bashar al-Assad for more than 10 days, according to activists.

It is estimated that about 80% of Homs' infrastructure has been destroyed by the shelling.

The violence came a day after nearly 170 people, mainly civilians, were killed in one of Syria's bloodiest days since a United Nations-backed ceasefire came into effect in April.

Meanwhile, the opposition said that at least four high-ranking officers defected from the Syrian army on Friday, a day after a fighter pilot landed his plane in neighbouring Jordan where he was granted political asylum.

France urged Syrian soldiers to follow the example of the defector, air force Col Hassan Merhi, praising his "courageous gesture".

Col Merhi "joins the growing number of Syrian officers, non-commissioned officers and military rank-and-file, who are choosing dignity and the struggle for freedom faced with the barbarity of the Syrian regime over the past 15 months", the French foreign ministry said in a statement.

Abu Alaa, a commander of the opposition Free Syrian Army, said an officer and his 30 soldiers who were besieging Homs also defected on Friday and joined the rebels.

In Geneva, a UN humanitarian organisation said the violence had raised the number of Syrians in need of humanitarian aid to 1,5-million.

"It's because we have been seeing ongoing relentless violence," said Jens Laerke, spokesman for the UN Office for the Co-ordination of Humanitarian Affairs.

He said aid organisations were having a difficult time reaching those who need food or shelter, despite a June 6 agreement by the Syrian government to let humanitarian agencies deliver $180m of aid.

"We don't have the access we need to implement that programme," Mr Laerke said.

The UN's World Food Programme has delivered food to 461000 Syrians this month and is planning to reach 850000 people in July.

The 16-month conflict has forced thousands of Syrians to flee into neighbouring countries including Lebanon, Turkey and Jordan.