Activists in Syria say shooting has broken out during a visit to a Damascus suburb by an advance team of United Nations observers.
The UN team is overseeing a shaky ceasefire brokered by UN-Arab League envoy Kofi Annan.
Meanwhile, the US Secretary of State Hillary Clinton has said Syria is at a turning point, where the violence must stop or the UUS will find ways to increase pressure on the Syrian government.
Speaking at a meeting of NATO ministers in Brussels, Mrs Clinton said President Bashar al-Assad faced tougher measures if he squandered his "last chance" by failing to implement Mr Annan's peace plan.
No formal agreement has been reached yet on how the new monitors should operate, and Syria challenged UN Secretary-General Ban Ki-moon over the size and scope of their mission.
Mr Ban said this week that 250 observers were insufficient in a country of 23 million where the UN says at least 9,000 people have been killed in the past 13 months. He sought European help in supplying planes and helicopters.
But Syrian Foreign Minister Walid al-Moualem said 250 was a "reasonable number", adding they should be from countries such as China, Russia, Brazil, India and South Africa - all more sympathetic to Damascus than are the West or the Arab League.
He also dismissed any need for UN aircraft.
At the same time, the Syrian army kept up its shelling of targets in Homs in violation of the ceasefire deal worked out with international envoy Kofi Annan. Explosions shook the battered Khalidiyah quarter and plumes of black smoke drifted over the rooftops.
