The countries, including Britain, France, Germany, Australia, Spain, Italy and Canada, expelled senior Syrian officials in what appeared to be a concerted effort to further isolate the Syrian government, The New York Times reported.
The United States expelled Syrian Charge d'Affairs Zuheir Jabbour, giving him 72 hours to leave the country, State Department spokeswoman Victoria Nuland said Tuesday morning.
The international action came as U.N.-Arab League special envoy Kofi Annan met with Syrian President Bashar Assad in Damascus, the Syrian capital.
A spokesman for Annan said the envoy urged the government to keep its commitment to honor the terms of a fragile peace plan engineered by Annan that included a cease-fire, political dialogue and freedom for Syrians to demonstrate.
"He conveyed in frank terms his view to President Assad that the six-point plan cannot succeed without bold steps to stop the violence and release detainees, and stressed the importance of full implementation of the plan," Annan spokesman Ahmad Fawzi said in a statement.
Annan will also meet with opposition leaders.
Also Tuesday, a U.N. High Commissioner for Human Rights spokesman said most of the 108 killed in Houla Saturday were executed.
Spokesman Rupert Colville said U.N. monitors found that fewer than 20 of the victims were killed by artillery fire in what the United Nations called an "outrageous show of force" by the Syrian authorities in Houla.
Colville said information indicated that entire families were killed in their homes.
Inside Syria, shops, including the Hamadiyah bazaar of Damascus, were closed as part of an opposition-led call to observe a three-day mourning period, the Times reported. Activists and residents said the government forced some stores in Damascus to reopen.
In Washington, Gen. Martin E. Dempsey, Joint Chiefs of Staff chairman, called the massacre "horrific" and "atrocious," saying military options were prepared in case the White House requested them, the Times said.
"There is always a military option, but that military option should always be wielded carefully," Dempsey said on Fox News Monday. "Because one thing we've learned about war, I have learned personally about war, is that it has a dynamic all its own -- it takes on a life of its own. [It] may come to a point with Syria because of the atrocities."
Lavrov reiterated that the position of Russia -- a trading partner with Syria -- was not tied to Assad's remaining in power but to Syrians steering their own political destiny.
"For us, the main thing is to put an end to the violence among civilians and to provide for political dialogue under which the Syrians themselves decide on the sovereignty of their country," he said.
The U.N. Security Council Sunday unanimously condemned the Houla massacre, in which 49 children and 34 women were among the dead. Stopping short of assigning blame, the council censured use of heavy artillery against civilians.
The Assad regime "categorically denied" responsibility for what it termed a "terrorist massacre."
The massacre could be pinned on the government with relative certainty because The New York Times reported that type of artillery is not in the hands of opposition fighters.
An Iranian general confirmed for the first time Iran had sent forces to Syria. Western nations accused Iran of providing military and technical support to Assad to quell protests since they began in March 2011 but Tehran said it provided only moral support to the Assad regime.