BEIRUT (AP) — 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 unravelling 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 to travel to Doha, Qatar, on Tuesday to brief the Arab League on the situation in Syria.

Diplomats and finance ministry officials from the Arab world, the West and elsewhere also were meeting Tuesday in Paris to coordinate sanctions against the Assad regime. 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.

A six-member advance team of U.N. observers arrived in Damascus over the weekend, but hasn’t travelled 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.

In Damascus, 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. “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.

While the overall level of violence is down since the cease-fire formally took effect Thursday, the regime has 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.

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.

Regime forces also fired mortars and shells at the neighborhoods of Khaldiyeh and Bayada in the central city of 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.

The regime appeared to be pushing to take control of the last rebel-held districts in Homs, said activists in the city. Khaldiyeh was shelled from three sides on Monday, and half of nearby Bayada fell under the army’s control over the weekend. Activists said Monday that the Free Syrian Army was holding its ground in the Qarabees and Jouret al-Shayah neighborhoods.

On Monday, at least five people were killed in Homs shelling, the Observatory said. Across Syria, at least 26 people were killed Monday, including 10 in a daylong gunbattle between rebel fighters and the Syrian army in the northwestern town of Idlib, the group said.

Another activist group, the Local Coordination Committees, said at least 55 people were killed Monday, including 26 in Idlib.