BEIRUT — The Syrian regime widened shelling attacks on opposition strongholds Tuesday, activists said, targeting a second town in a new sign that a U.N.-brokered cease-fire is quickly unraveling despite the presence of foreign observers.

The truce is part of an international plan to launch talks between President Bashar Assad's regime and those trying to topple him. An uprising against Assad erupted 13 months ago, but became increasingly violent in response to a regime crackdown.

Regime compliance with the cease-fire has been partial, and the latest escalation further lowered expectations that the key element of special envoy Kofi Annan's plan can stick. Mortar shells struck the central city of Homs at a pace of one a minute Tuesday morning, said Rami Abdul-Rahman, the head of the Britain-based Syrian Observatory for Human Rights, an activist group.

Annan, joint emissary for the U.N. and the Arab League, was briefing the Arab League in Qatar on the situation in Syria on Tuesday.

The envoy's plan has the backing of Syria's allies, including Russia, and despite setbacks is seen as the only way forward. Western military intervention is unlikely at this point, and economic sanctions, while starting to bite, seem insufficient to pressure the regime.

Leaders of two Syrian opposition groups said Tuesday, a day after meeting Russia's deputy foreign minister in Moscow, that they have sensed a shift in Russia's stance and hope Moscow will crank up pressure on Assad.

"Russia has all the necessary levers to apply pressure on Assad's government and help Annan's mission," said Haytham Manna of the Arab Commission for Human Rights, an activist group.

Russia twice shielded Assad from U.N. Security Council condemnation, but has become more critical of the regime.

In Paris, diplomats and finance ministry officials from the Arab world, the West and elsewhere were meeting Tuesday to coordinate sanctions against Syria. Diplomats say a string of EU, U.S. and other sanctions have affected Assad by curbing Syria's ability to export oil and the ability of his cronies to do business abroad.

In Syria, the overall level of violence is down since the cease-fire formally took effect Thursday, but the regime has gradually stepped up attacks. The number of people killed every day has also risen steadily since a brief lull that coincided with the start of the truce.

At least 26 people were reported killed on Monday, including 10 in a daylong gunbattle between rebel fighters and the Syrian army in the northwestern town of Idlib. Another activist group, the Local Coordination Committees, said at least 55 people were killed Monday, including 26 in Idlib.

A six-member advance team of U.N. observers arrived in Damascus over the weekend, but hasn't traveled to hotspots yet. U.N. officials said the team is still devising a plan on where to go and whom to meet. A previous Arab League observer mission was hampered by regime restrictions on movement, and U.N. chief Ban Ki-moon has demanded his monitors be given free access.

The head of the observer team, Col. Ahmed Himmiche, suggested Tuesday it would take time to get to the hardest hit areas. Work in Syria is "difficult," he said in Damascus. "There should be coordination and planning ... We must walk step by step as it's not an easy process."

The group is to be reinforced by an additional 25 monitors who are expected to arrive in the next few days, he said.

In violence Tuesday, army tanks shelled the southern town of Busra al-Harir, killing at least two people, according to the Observatory. The town, about 70 kilometers (45 miles) south of the capital of Damascus, is a stronghold of the rebel Free Syrian Army.

Adel al-Omari, an activist in the area, said troops have been shelling Busra al-Harir and the nearby rural region of Lajat since midday Monday. He said the shelling was intensifying and that many residents are fleeing to nearby villages or to Jordan.

Regime forces also fired mortars and shells at the neighborhoods of Khaldiyeh and Bayada in an apparent push to take control of the rebel-held districts in Homs, a center of the rebellion against Assad, according to the Observatory. Homs has been under continuous regime attack, with only a short break on the first day of the cease-fire, activists said.