With deaths mounting and the West expelling Syrian diplomats, international peace envoy Kofi Annan appealed to Syrian President Bashar al-Assad for “bold steps now, not tomorrow,” to push the U.N.'s six-point peace plan forward.

The U.N. special envoy told reporters in Damascus Tuesday that he conveyed to Mr. Assad the international community's concern about the bloodshed, including the massacre in Houla on May 25 of more than 100 men, women and children. He said the Syrian president also condemned the killings.

The U.N. human rights office in Geneva said Tuesday that U.N. monitors found that fewer than 20 of those killed in Houla were struck by artillery and tank fire. It said witnesses told the observers that most victims were killed in their homes in two waves of summary executions by pro-government militiamen.

U.N. peacekeeping chief Herve Ladsous told reporters in New York Tuesday that the use of heavy weaponry to kill some of the victims places responsibility “ever so clearly” on the Syrian government.

In response to the massacre, at least nine nations have ordered Syrian diplomats to leave.

French President Francois Hollande said he is expelling the Syrian ambassador in Paris in response to the massacre. Australia expelled two Syrian diplomats in a similar protest. The United States has given Syria's top diplomat in Washington, charge d'affaires Zuheir Jabbour, 72 hours to leave the country.

Britain, Germany, Italy, Spain, the Netherlands and Canada announced that they too will expel Syrian diplomats.

The Syrian government denies any role in the Houla killings and blames them on “armed terrorists” whom it accuses of driving a 15-month revolt against Mr. Assad.

U.N. observers continue to investigate last week's massacre. U.N. spokeswoman Sausan Ghosheh told VOA U.N. observers have remained in Houla since Saturday and have been talking to residents.

On Tuesday, Syrian rights activists said at least 19 people were killed across Syria in new attacks related to the uprising.

Mr. Annan mediated a cease-fire between government and rebel forces last month but it has failed to hold.

The special envoy also met with Syrian tribal leaders in Damascus Tuesday. Sheik Walid al-Sahu of the al-Roqeibat tribe told reporters that Syria is becoming more unstable.

“The implementation of his [Kofi Annan's] plan does not exist on the ground. We are witnessing more explosions and kidnappings.”

Syrian Deputy Foreign Minister Faisal al-Miqdad said Tuesday after meeting Mr. Annan that Syria has not committed a “single violation” of the cease-fire.

“We offered all that was necessary for the members of the [U.N.] mission to accomplish their job without any obstacles. The most important thing to be mentioned is that Syria, during this time, has not committed a single violation of Annan's plan nor the preliminary understanding signed between Syria and the United Nations.”

Russia's Foreign Minister Sergey Lavrov said Tuesday certain countries are starting to use the Houla massacre as a “pretext for putting forward demands for the necessity of taking military action.”

“Certain countries, as if the Security Council hadn't made a request for such an investigation, are starting to use these events, the events of May 25, as a pretext for putting forward demands for the necessity of taking military action, trying to put pressure on the U.N. Security Council.”

The United Nations says more than 10,000 people have been killed in Syria since the government began its crackdown on dissent in March 2011.