UNITED NATIONS - The U.N. peacekeeping chief said Tuesday that U.N. military observers in Syria are reporting cease-fire violations from the government and opposition, and he demanded an immediate halt to all violence.

Herve Ladsous refused to say which side was responsible for the most violations. But he said the unarmed observers had documented a number of Syrian heavy weapons deployed in populated areas - including armored personnel carriers and howitzers - despite the government's claim that it had withdrawn tanks and troops from cities and towns as required under international envoy Kofi Annan's peace plan.

Syrian forces fired mortar shells into a farming village near the Turkish border Tuesday, killing 10 people, among them two young children.

"The level of violence in Syria has been appalling," Ladsous said at a news conference. "I think the violations that are observed come from both sides. I would not establish a ratio. Now is not the time. . . . The important fact is that violations do come from both sides."

Twenty-four observers were in Syria on Tuesday in five locations - Damascus, Homs, Hama, Daraa and Idlib - all hot spots in the 13-month uprising, Ladsous said at a news conference. In each place, he said, they conduct mobile patrols during the day and sometimes at night.

He said the United Nations has commitments for about 150 observers, which are now being processed, with new pledges coming in daily, and he expects a rapid increase that will see the authorized total of 300 observers on the ground by the end of May.

But Ladsous said this requires Syria to give visas to the observers and it has already denied visas to three observers without reason. He declined to disclose their nationalities.

He said there were "verbal comments" from the Syrians about the Friends of Democratic Syria, which includes more than 70 nations. Among its members are the United States, many European countries, and a number of Mideast nations. President Bashar al-Assad's government said it would refuse visas to observers from the "Friends" group.

Ladsous said it is the U.N. peacekeeping department's responsibility to appoint observers and if Syrian authorities don't cooperate, "we report to the Security Council," as he did last week.

He said he expects the United Nations and Syria to sign an agreement "very rapidly" on the operation of the U.N. mission.

But Ladsous said Syria still refuses to allow the United Nations to use its own helicopters and air assets.