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."

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