Activists say Syrian government forces widened their attacks on opposition strongholds Tuesday, killing two civilians and wounding dozens more as a U.N.-brokered cease-fire continued to unravel despite the presence of foreign observers.

The Britain-based Syrian Observatory for Human Rights said the casualties occurred when army tanks shelled the town of Busra al-Harir, a stronghold of the rebel Free Syrian Army in southern Daraa province.

The activist group said troops loyal to Syrian President Bashar al-Assad also continued to shell the Khaldiyeh neighborhood in the central city of Homs, a center of the anti-government revolt. Homs has been under continuous attack, with only a short break on the first day of the cease-fire.

The casualties could not be independently verified.

In Damascus, the head of an advance team of six unarmed United Nations observers said it would take time for monitors to reach the hardest hit areas. Col. Ahmed Himmiche said Tuesday the group's mission “is a difficult process [that] requires coordination and planning…we should move step by step.”

An additional 25 monitors are expected to arrive within days.

The shaky cease-fire is part of international envoy Kofi Annan's six-point peace plan to launch talks between Mr. Assad's government and opposition forces attempting to oust him.

Mr. Annan travels to Doha, Qatar 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 are meeting Tuesday in Paris to coordinate sanctions against Damascus.

In Moscow, Russian Foreign Minister Sergei Lavrov accused unspecified external forces of seeking to undermine Mr. Annan's efforts to end more than a year of bloodshed in Syria, saying support for government foes is threatening the fragile cease-fire.

In televised remarks Tuesday, Lavrov said such actors “are doing this by delivering arms to the Syrian opposition and stimulating the activity of rebels who continue to attack both government and civilian facilities.”

Russia has provided Syria with weapons and – along with China – shielded Mr. Assad by blocking U.N. Security Council resolutions condemning his government for a crackdown in which the U.N. says its forces have killed more than 9,000 people since March 2011.

Moscow has pledged its full support for Mr. Annan's peace plan and last week called on the Syrian government to step up implementation, but Russia has also put much of the blame for the bloodshed on opposition forces.

On Monday, U.N. Secretary-General Ban Ki-moon said Damascus has a “responsibility” to ensure that observers on the ground in Syria can move freely to monitor the truce, which took effect last Thursday. Mr. Ban also urged Syrian security forces to exercise restraint and called on rebels to fully cooperate with the cease-fire, which he acknowledged is “very fragile.”

Mr. Annan has called for the monitoring team to be expanded to 250 personnel, but a second Council resolution is required for such a step.

The U.S. ambassador to the United Nations, Susan Rice, said Monday that if Syria's violence persists despite the cease-fire, “it will call into question the wisdom … of sending in the full monitoring presence.”

U.N. human rights investigators said Monday they have gathered evidence of Syrian government attacks on civilian areas since the truce began. The panel also said it continued to receive reports of human rights abuses committed by anti-government groups.

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