Britain says a Russian ship believed to be carrying munitions to Syria has apparently turned back after a British insurance company cancelled its insurance.

The Britain-based Standard Club insurer said Tuesday it was made aware of allegations that the cargo ship Alaed was carrying weapons destined for Syria and informed the ship's owners its insurance cover was revoked due to the nature of the voyage.

The move effectively halted the vessel, tracked off the northwest coast of Scotland, from continuing to its destination unless another insurer agreed to cover it. An EU arms embargo on Syria prevents insurers from covering the ship.

British Foreign Secretary William Hague told parliament Tuesday the ship appears to have turned back toward Russia.

Britain's Foreign Office said earlier in the day that it was aware a ship carrying refurbished Russian-made attack helicopters was en route to Syria. A Russian official said last week that Moscow had not sent new helicopters to Syria, but that it repairs helicopters it previously sold in order to fulfill contractual obligations.

Russia is a longtime ally of Syria and has shielded President Bashar al-Assad from U.N. sanctions sought by Western and Arab powers who oppose his 11-year autocratic rule.

The head of the U.N. monitoring mission in Syria briefs the U.N. Security Council in a closed session Tuesday. The work of the 300-strong observer team has been temporarily suspended due to escalating violence.

A senior U.N. official said U.N. Secretary-General Ban Ki-moon is “gravely concerned about the intensification of violence and rising death toll” in Syria. Clashes across the country have killed dozens since the observers suspended their work on Saturday.

Meanwhile, Syria says it is ready to act on a U.N. appeal to evacuate civilians from the central city of Homs, which government forces have bombarded since early June to try to crush a 15-month anti-government uprising.

The Syrian foreign ministry said Tuesday it had contacted U.N. observers in the country and local authorities to try to arrange an evacuation from Homs, where opposition activists estimate 1,000 families have been trapped. But the government said the observers' efforts failed due to obstruction by armed terrorist groups – its term for rebels leading the revolt. The Syrian statement also accused rebels of using Homs civilians as “human shields.”

U.N. observer chief Robert Mood has appealed to Syrian government and rebel forces to allow women, children and the wounded to flee Homs and other combat zones. The Britain-based Syrian Observatory for Human Rights has said dozens of wounded people are stuck in Homs and other rebel-dominated areas without medicine or doctors.

British Ambassador to the United Nations Mark Lyall Grant said Monday that many Security Council members will ask General Mood for his view of the mission's prospects for achieving its mandate in light of the conflict. In recent weeks, the unarmed observers have been caught up in several shooting and bombing incidents that damaged U.N. vehicles but caused no injuries to U.N. personnel.

The Security Council agreed to send the mission to Syria in April to monitor government and rebel compliance with a U.N.-backed cease-fire agreement, but the truce never took hold. The observers' 90-day mandate expires in mid-July. Grant said he does not rule out ending the mission before then.