BOGOTA (Reuters) - Colombia will reach agreement on a bilateral ceasefire at peace talks with leftist FARC rebels this week, President Juan Manuel Santos said on Tuesday, in what he said would mark a key advance in the negotiations to end 50 years of war.

Santos said this week the government and the Revolutionary Armed Forces of Colombia (FARC) rebels will complete the more than three-year-old negotiations by July 20.

"If the negotiators make a final effort to finish the definitive point that is a ceasefire and the end to hostilities, we will have taken a fundamental step in attaining peace," Santos said in a speech at an education event in Bogota.

"I appeal to God that he gives us the strength to finish these accords this very week, because we have almost completed them."

Government sources said the agreement would likely not mean the ceasefire would begin right away, but rather that the announcement would lay out the details of a ceasefire set to begin when a final peace deal is signed.

A ceasefire accord will likely include details on how the rebels will demobilize, the sources said, and FARC leader Rodrigo Londono, better known by his nom de guerre Timochenko, may sign the accord with Santos on Thursday or Friday.

The two sides have reached agreements on more than half a dozen topics but have yet to agree terms for the ceasefire or on how exactly a referendum for Colombians to approve the peace deal will be organized.

The FARC called a unilateral ceasefire nearly a year ago and the government responded by halting air strikes on rebel camps.

Negotiators missed a self-imposed deadline for signing a deal in March, and Santos has come under fire over the past week for comments about the referendum he has promised will take place to approve a deal.

Timochenko took to Twitter earlier on Tuesday to say he was against announcing a date by which the talks would finish.

"Practice has demonstrated that setting dates hurts the process, even more when there isn't an accord," the rebel leader tweeted. "Although we are advancing, we aren't there yet."

Latin America's longest war has killed some 220,000 people and displaced millions of others since 1964. Tens of thousands have gone missing.

(Reporting by Julia Symmes Cobb, additional reporting by Monica Garcia; Editing by Marguerita Choy)