SARAJEVO: Assistant Secretary of State Richard Holbrooke arrived in the Bosnian capital on Thursday, less than 36 hours before the first major deadline of the Dayton Peace accord, to assess the progress of the international peacekeeping mission and to help keep the heat on the warring parties to abide by the agreement he worked so hard to win. By midnight Friday, according to the treaty, all the warring parties are to have pulled their soldiers and weapons behind cease-fire lines and released all prisoners of war. Military observers in Bosnia agree that the demilitarization is proceeding on schedule. But there have been significant problems with the POW exchange. Bosnian Muslims are refusing to release prisoners until the Serbs give an accounting of more than 20,000 people, mostly civilians, who are missing. Most are presumed dead. Holbrooke, who has announced his intention to leave his job next month, will also fly to Belgrade for meetings with President Slobodan Milosevic.