The Arab League told Syria on Thursday that it has one day to agree to allow international observers into the country to monitor ongoing violence or it could face sanctions.

The organization has previously extended the deadline for the government to accept the observers’ mission but rejected Syria’s suggested changes to the plan. The sanctions could include freezing assets and stopping financial dealings.

The ultimatum comes amid mounting international pressure on the Syrian government to end a crackdown on a protest movement that has spread across the country since spring. Activists say that more than 400 protesters and security forces have been killed this month as the violence has increased.

French Foreign Minister Alain Juppe called Thursdayfor international forces to set up “humanitarian corridors” in Syria to allow aid groups access to cities where security forces’ operations have taken a heavy toll on civilians and sectarian tension is growing.

Although the European Union has no plans to expand sanctions, a spokeswoman said Thursday that “protection of civilians in Syria is an increasingly urgent and important aspect of responding to the events in country,” according to the Associated Press.

Turkey’s president, Abdullah Gul, and prime minister, Recep Tayyip Erdogan, have both called for Syrian President Bashar al-Assad, their former ally, to step down.

Some Syrian activists who had initially hoped to be able to pressure Assad’s government to reform or quit without international interference are now seeking outside help.

“We are inside the fire. If we go right, there will be fire, and if we go left, there will be fire,” said an activist in the eastern city of Deir al-Zour, who said the number of people attending demonstrations had increased since security ­forces fatally shot a 15-year-old two weeks ago.

“We think that the Arab governments should make a no-fly zone over Syria,” he said, speaking on the condition of anonymity because he fears for his safety.

However, the Brussels-based International Crisis Group think tank released a report Thursday expressing concern that Syria would be vulnerable to the many regional powers that have an interest in the outcome of the crisis there if international pressure or intervention forced Assad to step down.

The report suggests that the United States and Israel would try to shape a new leadership’s foreign policy and that Turkey might try to restrict the activities of Syria’s Kurdish minority and support the local branch of the Muslim Brotherhood. Iran, which has close ties with Assad, could sponsor the remnants of the security forces, it said.

“In short, the challenge is less to draw in outside actors than to keep them at bay,” the report said.