NATO chief Anders Fogh Rasmussen said Wednesday that foreign military intervention was "not the right path" in Syria despite the U.N.'s peacekeeping chief declaring the country in civil war.

Rasmussen said there were "no plans at this stage" for a NATO operation, as he condemned the U.N. Security Council failure to reach agreement as a "big mistake", saying Russia could have an "instrumental role" in brokering peace.

Syria is now in a full-scale civil war, U.N. peacekeeping chief Herve Ladsous said, as the United States voiced fears the U.N. mission in the country may not survive once its mandate expires in July.

World powers are groping to find a way to end the bloodshed in Syria with the toll growing daily despite a ceasefire that should have gone into effect from April 12, and amid reports of children being used as human shields.

At least 73 people were killed in shelling and clashes across Syria on Tuesday, 61 of whom were civilians and rebels and 12 regime soldiers, according to the opposition Local Coordination Committees and the Syrian Observatory for Human Rights.

Regime forces rained shells on rebel positions in the northwestern Latakia province, pounding the town of al-Haffe for the eighth straight day as they appeared to be preparing to storm it, monitors said.

Tunisia said Tuesday it has imposed a dusk-to-dawn curfew in eight regions of the country including the capital Tunis after rioting blamed on ultra-conservative Salafist Muslims.

The curfews take effect from 9 pm to 5 am and cover areas in the southeast and northeast of the country including the greater Tunis region, the defense and interior ministries said in a joint statement.

Secretary of State Hillary Clinton warned that Washington would find it hard to support the extension of the observer mission beyond next month if the government of President Bahar al-Assad's regime continued to show "contempt" for a U.N.-backed ceasefire that was supposed to go into effect from April 12.

Asked whether he believed Syria is in a civil war, Ladsous told reporters: "Yes I think we can say that. Clearly what is happening is that the government of Syria lost some large chunks of territory, several cities to the opposition, and wants to retake control.

Syria on Tuesday accused Washington of encouraging more massacres in the strife-torn country and of meddling in its internal affairs.

"The U.S. administration is pushing forth with its flagrant interference in Syria's internal affairs and its backing of armed terrorist groups," the foreign ministry said in a statement.

The U.N. Security Council on Tuesday unanimously passed a resolution threatening sanctions against groups seen undermining Yemen's political transition and staging attacks in the country.

So-called "spoilers" such as the family and supporters of former president Ali Abdullah Saleh, though not named in the resolution, are a particular target of the warning in Resolution 2051, diplomats said.

Six far-right militants were arrested Tuesday after four Egyptians were assaulted in an industrial suburb of Athens, police said, amid an increase in attacks on immigrants in the crisis-hit nation.

A local Muslim leader accused Greek politicians, particularly those on the right, of stoking racist violence ahead of a general election on Sunday that could determine whether the country will stay in the eurozone.

Iran on Tuesday welcomed a Russian proposal on holding an international conference on Syria aimed at saving a faltering peace plan for that country's conflict.

"We support any method which helps positive talks between the government and the opposition in an appropriate environment, and also suitable reform actions for realizing the people's ambitions," Iranian foreign ministry spokesman Ramin Mehmanparast told reporters at a weekly press briefing.

Pro-Syrian regime villagers on Tuesday prevented United Nations observers from reaching the town of Al-Haffe where there are fears that a new massacre might be carried out, a watchdog said.

"Residents of the pro-regime village of As-Sheer blocked the road and prevented the U.N. observer team from reaching Al-Haffe," the Syrian Observatory for Human Rights said.