China, which has also vetoed international efforts to isolate Syria, criticized the new conditions. Syria has undermined every truce plan it ostensibly accepted since the uprising started in March 2011.

Mr. Annan considered the plan still alive and was waiting to see what Syria did this week, said Ahmad Fawzi, his spokesman. “To say now that you want written guarantees is a condition that was never in the agreement,” Mr. Fawzi said. “The deadline is still valid, and the plan is still on the table.”

Image Border shootings were deadly in Turkey and in Lebanon. Credit... The New York Times

Mr. Annan was scheduled to head to Turkey on Tuesday to visit some of the nearly 25,000 Syrian refugees there, several thousand of whom arrived in recent days, before moving on to Iran to try to persuade another important Syrian ally to back his plan. Senators John McCain of Arizona and Joseph I. Lieberman of Connecticut were also scheduled to meet with refugees in Turkey on Tuesday.

The Syrian National Council, the main opposition umbrella group, said it was ready to respect a cease-fire.

“The opposition, including the Free Syrian Army, says it will comply if the regime does,” said Bassma Kodmani, a member of the Syrian National Council’s executive committee, adding that opposition groups within Syria were ready to put a cease-fire into effect on Tuesday and not wait until Thursday. She called the government’s actions “clearly not a sign of their intention to cooperate in good faith.”

In another ominous sign, deadly violence has intensified in the days leading up to the cease-fire deadline, with opposition groups reporting scores killed on Monday alone in besieged cities and towns across Syria. In Aleppo, which has been less turbulent than much of the country, 10 security officers were shot dead quelling a demonstration and 11 were wounded, along with several civilians, the official Syrian Arab News Agency reported. It said 25 security men were buried nationwide Monday.