The International Committee of the Red Cross (ICRC), which said it was postponing an aid convoy that had been set to deliver supplies to four besieged Syrian towns, warned of the consequences for millions of civilians in need.
"As an immediate security measure, other convoy movements in Syria have been suspended for the time being pending further assessment of the security situation," Jens Laerke, U.N. humanitarian aid spokesman, told a news briefing in Geneva.
"However we remain committed to stay and deliver to everybody in need in Syria," he added.
Syrian or Russian aircraft struck an aid convoy near Aleppo on Monday, a war monitor reported, as the Syrian military declared a one-week truce over.
The confirmed death toll on Tuesday stood at one Syrian Arab Red Crescent (SARC) staff member and "around 20 civilians", the SARC and Red Cross said in a joint statement.
"If this callous attack is found to be a deliberate targeting of humanitarians, it would amount to a war crime," U.N. aid chief Stephen O'Brien said in a statement calling for an immediate and independent investigation.
The United Nations had just received permission from the Syrian government to deliver aid to all besieged areas in the country, Laerke said. All parties, including Russia and the United States, had been notified about the cleared convoy heading to rebel-held eastern Aleppo.
"Yesterday's attack is a flagrant violation of international humanitarian law and it is unacceptable," Peter Maurer, president of the International Committee of the Red Cross (ICRC) said in a statement. "Failing to protect humanitarian workers and structures might have serious repercussions on ongoing humanitarian work in the country, hence depriving millions of people of aid essential to their survival."
Aid convoys for four Syrian towns will be postponed as staff reassess security after the deadly attack on relief trucks and intensified violence, a senior ICRC official said earlier.
"This is very worrying. We see a resumption of violence, an intensification of fighting in many locations," Robert Mardini, ICRC director for the Middle East and North Africa, told Reuters in Geneva.
"We had something planned in the four towns, but for now it is put on hold to reassess the security conditions," he said, referring to rebel-besieged Foua and Kefraya in Idlib and government-blockaded Madaya and Zabadani near the Lebanese border.
At least 18 of 31 trucks in a U.N. and Syrian Arab Red Crescent (SARC) convoy were hit on Monday along with an SARC warehouse. The convoy was delivering aid for 78,000 people in the hard-to-reach town of Urm al-Kubra in Aleppo province.
"Life-saving aid supplies have been totally damaged and a health clinic destroyed, depriving thousands of civilians of much needed food and medical assistance," said Benoit Carpentier of the International Federation of Red Cross and Red Crescent Societies.
SARC's director in Urem al-Kubra, Omar Barakat, was among the dead, Mardini said. "The team is in shock."
"Omar was badly injured and the rescue team could not reach him for two hours. When he was evacuated he could not survive his wounds," he said.
A separate SARC/ICRC convoy to Talbiseh in Homs province made its first delivery since July on Monday, carrying supplies for more than 80,000 people.
"It is difficult to read the environment in coming hours because you have a mixture of intensification of fighting and politicization of humanitarian aid ... It is high time to de-link humanitarian work from politics," Mardini said.