Syria is still preventing the Red Cross from reaching thousands of stranded civilians in Homs as government troops continue shelling several areas of the city.

Opposition activists reported heavy mortar and machine gun fire in the districts of Jobar, Khaldiyeh, Bab Sbaa, Khader and Qusoor early on Saturday.

Syrian Network for Human Rights said forces loyal to President Bashar al-Assad were bombarding Jobar, where thousands of civilians have taken refuge after fleeing from the neighbouring Baba Amr, "in an act of pure revenge".

"Assad's army has been firing mortar rounds and 500mm machine guns since this morning at Jobar. We have no immediate reports of casualties because of the difficulty of communications," it said in a statement.

Abu Hassan al-Homsi, a doctor at a makeshift clinic in Khaldiyeh district, said he treated a dozen wounded people. "This has become routine, the mortars start falling early in the morning," he said.

The shelling comes amid a standoff between the Syrian regime and the International Committee of the Red Cross (ICRC), which says authorities have prevented its convoy from delivering badly needed food, medical supplies and blankets to thousands of people still stranded in Baba Amr, the rebel-held district of Homs that was overrun by troops on Thursday.

The Red Cross said it was still negotiating access for its seven trucks, as well as Red Crescent ambulances.

"The ICRC and Syrian Red Crescent are not yet in Baba Amro today. We are still in negotiations with authorities in order to enter Baba Amro. It is important that we enter today. We are not about to give up," an ICRC spokesman, Hicham Hassan, told Reuters on Saturday.

The refusal to allow the convoy in to treat and evacuate the wounded came as western diplomats received the bodies of Sunday Times journalist Marie Colvin and French photographer Rémi Ochlik, who were killed in an attack on a press centre in Baba Amr more than a week ago.

A Reuters witness said the diplomats, believed to be the French ambassador to Syria, Eric Chevalier, and a representative from the Polish embassy, which is managing US affairs in Syria, had taken the bodies from the Al-Assad University Hospital in Damascus.

The French journalist Edith Bouvier, who was badly wounded in the same attack, arrived back in France on Friday with her colleague William Daniels.

The UN secretary-general, Ban Ki-moon, said on Friday that he had received "grisly reports" of Assad's forces arbitrarily executing, imprisoning and torturing people in Homs after the rebel Free Syrian Army withdrew from the city.

Anti-Assad activists have accused Syrian troops of burning houses, arresting all males over 12 and of extrajudicial killings, including the alleged beheadings of 17 men captured after Baba Amr's fall.

The wounded British photographer Paul Conroy, who escaped from Homs earlier this week, said he had witnessed Syrian troops carrying out a massacre in the district.

Elsewhere in Syria, state media said two people had been killed in a suicide car bomb attack in the city of Daraa, known as the birthplace of the country's uprising.

The Sana state news agency said the bombing occurred at a roundabout in the central Daraa al-Balad district of the city, causing multiple casualties and damaging buildings.

The British-based Syrian Observatory for Human Rights said at least two people were killed and several others wounded in the explosion – the first suicide bombing attack on an opposition stronghold.

The charity Save the Children on Saturday called for more international pressure to be brought on Syria to stop the fighting, to ensure aid agencies could help the thousands of children caught up in the conflict.

Justin Forsyth, the charity's chief executive, said: "We know children have already lost their lives, and others have been badly wounded. Children in Syria will be deeply scarred by their experiences and need urgent help.

"We need people around the world to join us in calling for an immediate end to the violence so we can get urgent help to children in Syria."

France said on Friday it is closing its embassy in Syria. The US and Britain have already closed their embassies.

The French president, Nicolas Sarkozy, called the events in Syria a scandal, adding that the European Council "condemned in the harshest terms what is happening in Syria".

The west has stepped up its criticism of Assad's regime amid mounting reports of atrocities by his security forces. The US has called for him to step down and the US secretary of state, Hillary Clinton, has said he could be considered a war criminal.