U.S. Secretary of State Hillary Clinton has called for tough U.N. sanctions to be imposed on Syria if it fails to implement a U.N.-backed peace plan aimed at resolving the country's year-long uprising.

Clinton made the call Thursday at a Paris meeting of top diplomats of more than a dozen nations who call themselves the “Friends of Syria” and support the Syrian opposition revolt against the autocratic rule of President Bashar al-Assad.

Clinton said Washington believes the U.N. Security Council should adopt an arms embargo on Syria and impose travel and financial sanctions on Syrian officials in order to keep Mr. Assad “off balance.” She also said such a resolution should be passed under Chapter Seven of the U.N. Charter, which authorizes measures, including military action.

Russia has twice used its Security Council veto to block Western and Arab proposals for sanctions on the Assad government, a key Russian ally. Clinton said she tried to persuade Russia to drop its objection to sanctions in a meeting earlier Thursday with her Russian counterpart, Sergei Lavrov, in Brussels. She said Lavrov recognized that Syria is “not in a static situation but a deteriorating one.”

French Foreign Minister Alain Juppe said the Friends of Syria believe the U.N.-backed peace plan drafted by international envoy Kofi Annan is the “last chance” for Syria to avoid a civil war. He said if the plan fails, the anti-Assad coalition will seek “other options” to deal with the Syrian crisis.

The United Nations said Syria signed a preliminary agreement for an expanded U.N. mission to monitor a shaky cease-fire in the government's conflict with rebels leading the uprising. A small group of unarmed U.N. soldiers began monitoring the truce earlier this week. The cease-fire took effect last Thursday, but fighting has continued across the country.

U.N. Secretary-General Ban Ki-moon urged the Security Council to quickly adopt a resolution expanding the observer team in Syria from about 30 to 300 personnel. He also called on Syria to cooperate with the monitors by ensuring their freedom of movement, including through the use of helicopters.

The U.N. chief described the situation in Syria as “highly precarious,” citing recent reports of government shelling of civilian areas and attacks by armed opposition groups.

Syrian rights activists said security forces and rebels fought in the eastern town of Deir el-Zour on Thursday. They also reported intense government artillery strikes on the opposition hub of Homs in central Syria.

Elsewhere, the head of the small U.N. observer team in Syria visited the southern city of Daraa, another center of protest against Mr. Assad. Amateur video showed Moroccan Colonel Ahmed Himiche visiting homes damaged in the conflict and listening to residents asking for information about the fate of missing relatives. At one point, he responded by saying “I am a military man, not an expert…I will look with my eyes and write my report.”

U.S. Defense Secretary Leon Panetta said the Syrian government's crackdown on the opposition is “brutal and devastating.” In a briefing to U.S. lawmakers, he said the United States is supporting international efforts to curb the violence in several ways, including by providing direct non-lethal support to the Syrian opposition.

Panetta also reiterated the Obama administration's opposition to military intervention in Syria, saying that could make a “volatile situation even worse” and “place more innocent civilians at risk.”

A U.N.-backed survey released on Thursday said 230,000 Syrians have been displaced in the country since the uprising began in March 2011.

The United Nations estimates that Syria's crackdown on the revolt has killed more than 9,000 people.