UNITED NATIONS, (Reuters) – The U.N. Security  Council yesterday voiced grave concern for the first time  over civilian deaths in Sri Lanka’s war and demanded that the  army and rebels take urgent action to protect civilians.

The message to the Tamil Tiger rebels and government of Sri  Lanka was agreed on during a closed-door meeting of the  15-nation Security Council, its first formal consultations on  the issue since the Sri Lankan war escalated sharply several  months ago.

“The members of the Security Council express grave concern  over the worsening humanitarian crisis in northeast Sri Lanka,  in particular the reports of hundreds of civilian casualties in  recent days,” the unanimously agreed statement said.

Shelling killed a Red Cross worker inside Sri Lanka’s war  zone yesterday and hundreds of civilians were reported  killed on Sunday and Monday by artillery fire as troops  attacked a narrow strip of land held by Liberation Tigers of  Tamil Eelam, or LTTE, rebels, who look set to lose a 25-year  war.

Diplomats said the heavy civilian deaths reported over the  weekend were one of the factors that helped to convince Russia,  China, Vietnam and Libya that it was necessary to have a formal  meeting on the situation in Sri Lanka.

Although the council had held several informal meetings on  the war, the four countries had opposed any formal council  action on what they see as an internal Sri Lankan matter. While  it is not legally binding, council diplomats said the unanimous  statement would help to ratchet up the pressure on Sri Lanka.

“This is an important step forward by the Security  Council,” British Ambassador John Sawers said. “We have for the  first time produced an official written statement by the  council addressing the worsening humanitarian crisis in Sri  Lanka.”

In Washington, U.S. President Barack Obama called for both  sides to prevent a humanitarian disaster for the tens of  thousands of people trapped in the war zone.

“Without urgent action this humanitarian crisis could turn  into a catastrophe,” Obama said at the White House.

The Security Council statement said the council members  “strongly condemn the LTTE for its acts of terrorism over many  years” and urged the group to “lay down its arms and allow the  tens of the thousands of civilians to leave.” U.N. officials  say the rebels are using civilians as human shields.

The Security Council also voiced “deep concern” about  reports of continued heavy shelling by government forces in the  conflict zone — reports that the Sri Lankan government has  repeatedly denied.

The statement, which was drafted by France, Britain and  Austria with the backing of the United States, also expressed  support for the “personal involvement” of U.N.  Secretary-General Ban Ki-moon, whom the government has invited  to go to Sri Lanka.

Ban said yesterday he was appalled by the reports of heavy  civilian casualties.

Council members agreed earlier on Wednesday to issue a  brief statement expressing their “concern” over a U.N. board of  inquiry report about Israel’s January war against Hamas  militants in the Gaza Strip that was critical of Israel.