Opposition groups in the cities of Idlib in the north, central Homs and Deraa in the south said the army had hidden armor in dugouts and replaced tanks with blue armored vehicles said to belong to police forces.

An Arab League mission arrived in Syria last week to verify whether the government was implementing a peace plan under which it had agreed to withdraw its military presence from restive areas and to release thousands of prisoners detained since the uprising against President Bashar began last March.

League Secretary General Nabil Elaraby said in Cairo on Monday the monitors had reported back that state forces had withdrawn from residential areas. The mission was ensuring a halt to bloodshed and had secured the release of about 3,500 prisoners, he said.

"We are not seeing the release of detainees or the true removal of a military presence from the streets," said Rami Abdelrahman, head of the British-based Syrian Observatory for Human Rights. "Army tanks have been replaced with police armored personnel carriers that still have the capability to shoot heavy weaponry."

Videos uploaded by activists on the Internet showed armored vehicles hidden behind high dirt barriers.

"Nabil Elaraby, you are in Cairo and we're in Baba Amr. Here are the tanks and there are your monitors," said one activist in a video uploaded on the Internet which showed a team of orange-vested men who appeared to be League monitors standing near an armored vehicle behind a barrier.

Activists also accused the regime of misleading Arab League observers by taking them only to areas loyal to the government and changing street signs to confuse them.

Foreign Ministry spokesman Jihad Makdissi denied the charges and said the government does not interfere with the observers, "but rather ensures protection and escort for them."

The Syrian government on Wednesday rejected accusations from Washington that it was failing to live up to its agreement with the Arab League. Washington accused the government was trying to stoke more violence to justify retaliation.

"Such a statement is offensive to the Arab League ... because it is a blatant interference in the core of its work, the sovereignty of its states and an unjustified attempt to internationalize (the Syria crisis)," Foreign Ministry spokesman Makdesi said in a statement. "This is a pre-emptive stance that damages the performance of the Arab monitors' mission prior to their first initial report."

The mission is expected to report on its first week of work by end of this week.

Activists said the monitors visited the main prison in Homs, a hotbed of protest. They were greeted with chants of "freedom, freedom" and "the people want the fall of the regime", the activists said, citing leaked messages from detainees and prison guards.

"The security forces moved some of the detainees but the numbers are so huge they couldn't move them all," said Homs activist Abu Rami. "They moved some detainees to military bases and moved away the ones who are in bad health."

Under the agreement to allow in the monitors, the mission cannot enter military sites.