Syria called local truces near Damascus and in a northern province on Friday but no halt to combat on the main battlefield in Aleppo, after a surge in fighting the United Nations said showed "monstrous disregard" for civilian lives.

A new "regime of calm" would begin from 1am on Saturday and last one day in the capital's eastern Ghouta suburb and three days in the northern countryside of the coastal province of Latakia, the army said in a statement.

But by excluding the city of Aleppo, scene of the worst recent violence, the narrow truces were unlikely to resurrect a ceasefire and peace talks that have collapsed this week.

In the worst recent attack, an air strike destroyed a hospital in a rebel-held area overnight on Wednesday-Thursday. The French charity Medecins sans Frontieres, which supported the hospital, said on Friday the death toll had risen to at least 50, including six medics.

A Syrian military source said Aleppo was excluded from the newly announced truces "because in Aleppo there are terrorists who have not stopped hitting the city and its residents ... There are a large number of martyrs in Aleppo, which is why the situation is different there".

Russia's Interfax news agency quoted the officer in charge of a Russian ceasefire monitoring centre as saying the truces meant all military action would cease in the covered areas.

Damascus described the truces as an attempt to salvage a wider "cessation of hostilities" agreement in place since February. That ceasefire, sponsored by Washington and Moscow, allowed peace talks to start but has all but completely collapsed in recent days along with the Geneva negotiations.

Violence was "soaring back to the levels we saw prior to the cessation of hostilities," said United Nations human rights chief Zeid Ra'ad al-Hussein.

"There are deeply disturbing reports of military build-ups indicating preparations for a lethal escalation," Zeid said in a statement that described a "monstrous disregard for civilian lives by all parties to the conflict".

"I dread more horror"

The United Nations has called on Moscow and Washington to help restore the ceasefire to prevent the collapse of peace talks, which broke up this week in Geneva with virtually no progress after the opposition walked out.

"The cessation of hostilities and the Geneva talks were the only game in town, and if they are abandoned now, I dread to think how much more horror we will see in Syria," Zeid said.

The United States said on Friday that it was in discussions with Russia to renew the cessation of hostilities and was seeking a halt to fighting in Latakia and eastern Ghouta as a test case before trying to extend ceasefires throughout the country.

"We are in touch with the opposition and it is our expectation they will comply," State Department spokesman Mark Toner said of the Latakia and Ghouta ceasefires.

Asked why the United States did not try to get a halt to the violence in Aleppo, Toner said, "In part it is a recognition that Aleppo is very complex and the fighting around there is indeed alarming. We need to start somewhere and we're going to start with Latakia and east Ghouta."

Aleppo, Syria's largest city before the war, has been divided for years between rebel and government zones. Full control would be the most important prize for President Bashar al-Assad, who has been fighting to keep hold of his country throughout a five-year civil war. U.N. mediator Staffan de Mistura has said that up to 400,000 people have been killed.

Since Russia joined the war last year with air strikes against Assad's enemies, battlefield momentum has shifted in the government's favor.

Hundreds of thousands of people still live in rebel zones of Aleppo, and the countryside to the north includes the only stretch of the Turkish-Syrian border still in the hands of Assad's main opponents, Arab Sunni Muslim rebel groups.

Opposition groups have accused the government of deliberately targeting civilians so they would abandon the area.

"The aim of what is happening in Aleppo now is to put pressure on us to accept the smallest demands and conditions proposed by Bashar al-Assad," chief opposition negotiator Asaad al-Zoubi said in an interview with Al Jazeera television.

"The truce which they are talking about today is so the regime can prepare its forces again, especially because it failed to take control of Aleppo."

The leader of the opposition High Negotiations Committee, Riyad Hijab, wrote to U.N. Secretary General Ban Ki-moon accusing the government of "horrifying daily massacres" and "the besiegement and starvation of cities, towns and villages". He demanded the U.N. Security Council force Assad to stop.

Rebels have targeted government areas with aggressive shelling, which Damascus has said is proof that they are receiving weapons from abroad.

Air strikes, shelling

Commenting on the hospital air strike, International Committee of the Red Cross spokesman Ewan Watson told Reuters in Geneva, "It is unacceptable ... But it is up to an investigator and it is for a court to take that decision on whether it is a war crime or not."

Late on Friday, the organization issued a statement calling for an immediate halt in the attacks, saying another four medical facilities on both sides of the frontlines in Aleppo had been damaged extensively. It said dozens of people had died and were injured in "another day of relentless fighting in Aleppo today."

It said the four healthcare facilities that had been hit were Al Marjah polyclinic, the Bustan Al Qassar polyclinic and Shawki Hill Dialysis Centre and the Ibn Rashid Cardiac Hospital.

"There can be no justification for these appalling acts of violence deliberately targeting hospitals and clinics, which are prohibited under International Humanitarian Law," Marianne Gasser, head of the International Committee of the Red Cross in Syria, said in the statement.

"People keep dying in these attacks. There is no safe place anymore in Aleppo. Even in hospitals. For the sake of people in Aleppo, we call for all to stop this indiscriminate violence."

The Syrian Observatory for Human Rights, a British-based monitoring group, said air strikes and government shelling had killed at least 142 civilians including 21 children in rebel areas in the past eight days, while rebel shelling of government areas had killed 84 civilians including 14 children.

The Observatory said at least 11 civilians were killed on Friday in rebel areas and 13 in government areas. In the rebel-held zones, more were trapped under fallen buildings destroyed in helicopter strikes.

Bebars Mishal, a civil defense chief working in rebel-held areas of Aleppo, told Reuters there were a number of air attacks in the morning, many of them around mosques in rebel-held areas. Mishal said one hit a clinic in Aleppo's Al-Marja district.

Syrian state media said a number of people had been killed and wounded and fires started during shelling of government-held quarters in Aleppo, which included a hit on a mosque as people were leaving Friday prayers.