The U.N. political chief says the Syrian government is still using heavy weapons and has failed to fully implement the six-point peace plan which President Bashar Assad's government approved.

B. Lynn Pascoe told the U.N. Security Council on Monday that the cease-fire remains incomplete and "human rights violations are still perpetrated with impunity."

"We are at a pivotal moment in Syria," Pascoe said.

Pascoe demanded the immediate withdrawal of weapons and troops from cities and towns. Thousands have been killed in more than a year of protests against the Syrian regime.

Pascoe said Syria's compliance with other elements of the peace plan, including releasing detainees and allowing peaceful demonstrations, are "clearly insufficient."

Pascoe said the U.N. hopes an expanded observer force will help stop the killing and create conditions for a "credible political process."