Russian air strikes Saturday on an Al-Qaeda run prison in Syria killed nearly 60 people, a monitor said, as aid needed in three besieged towns where people are reportedly starving was delayed.

The bloodshed came as UN envoy Staffan de Mistura met in Damascus with Syrian Foreign Minister Walid Muallem to prepare for peace talks between the government and the opposition.

The Syrian Observatory for Human Rights said the air raids that hit a building near a popular market in the northwestern Idlib town of Maarat al-Numan killed 21 civilians, 29 militants and seven detainees.

Thirty people were wounded, with many in critical condition.

The building houses a jail and a religious court run by Al-Nusra Front, Al-Qaeda's affiliate in Syria, said the Britain-based Observatory which relies on a network of sources in the ground.

A child and two women were among the civilians who died, while 23 Nusra fighters were among the militants killed.

Russian warplanes have been conducting strikes against the Islamic State organisation and other "terrorist" groups in Syria since September 30.

Although Al-Nusra and IS are both jihadist organisations, they are fierce rivals and regularly clash in Syria.

But Al-Nusra has formed an alliance -- dubbed the Army of Conquest -- with other rebel groups in Idlib, including the hardline Ahrar al-Sham.

Aid deliveries to Madaya, a town near Damascus surrounded by the army, and to Fuaa and Kafraya which are encircled by rebels in Idlib province had been expected Sunday.

But the Red Cross said the much-needed supplies cannot be delivered before Monday due to logistical problems.

- 'Logistical issues' -

"The distribution of aid will not take place on Sunday for logistical reasons; we are working hard for it to take place on Monday," said Pawel Krzysiek, spokesman in Damascus of the International Committee of the Red Cross.

Tamam Mehrez of the Syrian Red Crescent had said "technically we are ready to begin distribution as early as Sunday, but if there are logistical issues it will be Monday at the latest".

Madaya, home to 42,000 people, has become notorious in recent days because of people starving in the town.

It has been surrounded by regime troops for six months.

The Syrian government agreed Thursday to allow aid into Madaya as part of a deal that will see aid simultaneously reach 20,000 people trapped in Fuaa and Kafraya.

According to Doctors Without Borders, at least 23 people have starved to death since December 1 in Madaya.

The UN Security Council is to discuss the matter behind closed doors on Monday, although no decision is expected.

More than 260,000 people have been killed since the Syrian conflict erupted in March 2011 and millions have been forced from their homes but a solution to end the bloodshed has been hard to nail.

The UN envoy has been on a regional tour to shore up support for peace talks due to take place in Geneva on January 25 between the Syrian government and its opponents.

The talks are the first step in an ambitious 18-month plan, endorsed by the UN, to bring about a political transition in Syria.

After talks in Riyadh this week, De Mistura met Muallem in Damascus on Saturday and is next planning on visiting Tehran.

De Mistura's office said the envoy had a "useful" meeting with the Syrian foreign minister.

Muallem has confirmed his government would take part in the Geneva talks but was still waiting to receive the names of opposition figures who would also attend, the official SANA news agency said.

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