Air raids have hit four makeshift hospitals in Syria's battered Aleppo city, doctors said Sunday, jeopardizing medical care for more than 200,000 desperate civilians in rebel-held areas.

The bombardment since Saturday has worsened the plight of residents of besieged eastern neighborhoods of Syria's second city, where food and medical supplies are becoming increasingly scarce.

Nearly 40 Syrian soldiers and pro-regime fighters were killed when rebels blew up a tunnel under a government position in Aleppo city, a monitor said Friday.

At least 43 civilians, including 11 children, were killed in bombardment of several rebel-held areas across Syria on Thursday, the Syrian Observatory for Human Rights monitor said.

Air raids on rebel-held districts of Syria's battleground second city of Aleppo killed 18 civilians on Saturday, a monitor said.

"Eleven civilians, including four children, were killed by air raids after midnight in the Bab al-Nasr area of Old Aleppo, and seven others, including a baby, were killed in Fardous neighborhood," the Syrian Observatory for Human Rights said.

When the farmers of Syria's Tartus province went off to war, their famed tomato vines were left to wither.

Syria's army has extended a nationwide truce for another three days, but continued Tuesday to press its campaign against rebels in the battered northern city of Aleppo.

US Secretary of State John Kerry will make his latest trip to Moscow this week to seek common ground on how to deal with the ongoing bloodshed in Syria.

The State Department said Kerry would speak with senior Russian officials about Syria and Ukraine, as well as tensions between Armenia and Azerbaijan over the Nagorny-Karabakh region.

At least 29 Syrian rebels were killed in clashes with regime troops overnight during a failed bid to reopen the opposition's key supply route into Aleppo city, a monitor said Sunday.

The fighters from the Faylaq al-Sham Islamist faction and Al-Qaeda affiliate Al-Nusra Front were killed trying to reopen the Castello Road, which regime forces effectively severed on Thursday, the Syrian Observatory for Human Rights monitor said.

A child was among at least 15 civilians killed in air strikes on an al-Qaida-held town in northwestern Syria on Friday, a monitoring group said.

The Syrian army said Wednesday it would observe a 72-hour ceasefire across the country, in an announcement coinciding with the festival marking the end of the Islamic holy month of Ramadan.