BEIRUT (Reuters) - International mediator Kofi Annan said on Monday he was horrified by the killings in the Syrian town of Houla and urged the Syrian government to take bold steps to show it was serious about reaching a peaceful solution to the country's crisis.

Speaking shortly after arriving in Damascus, Annan said he expected to have "serious and frank discussions" with President Bashar al-Assad. The two men are due to meet on Tuesday, according to the Syrian Foreign Ministry.

Annan said the killing of 108 people in Houla was "an appalling crime, and the (U.N.) Security Council has rightly condemned it". Western countries have blamed Assad's forces for the killings, a charge Damascus denies.

"I urge the (Syrian) government to take bold steps to signal that it is serious in its intention to resolve this crisis peacefully, and for everyone involved to help create the right context for a credible political process," Annan told reporters on his arrival in Damascus.

"And this message of peace is not only for the government, but for everyone with a gun".

Annan brokered an April 12 ceasefire agreement, part of a broader six-point plan aimed at ending the bloodshed which has engulfed a 14-month uprising against Assad and leading towards a negotiated solution. But the truce has not held and hundreds of people have been killed in the last six weeks.

"The six point plan has to be implemented comprehensively, and this is not happening," Annan said.

(Reporting by Dominic Evans, editing by Diana Abdallah)