MOSCOW, May 26 (UPI) -- Russia's special envoy for Yugoslavia, Viktor Chernomyrdin, is pressing for an immediate halt to NATO's bombing of Yugoslavia in order to give the political process a chance to succeed. Chernomyrdin, speaking before the resumption of trilateral talks in Moscow with U.S. Deputy Secretary of State Strobe Talbott and Finnish President Martti Ahtisaari, the third such meeting in two weeks, said he hoped concrete results would be reached during the negotiations. Chernomyrdin told reporters he expected the talks would result in ``an end to the airstrikes on Yugoslavia, which will make it possible to put the military conflict on a political footing.'' However, despite Russia's push for a quick end to the bombings, there was no sign that an agreement on a cease-fire would be reached today. As the talks began, White House spokesman David Leavy told United Press International the Clinton administration is ruling out an immediate halt to the bombing of Yugoslavia until all of NATO's demands are met by Belgrade. Leavy said the demands must be met in ``a clear, verifiable and demonstrable'' way before the bombing stops. Said Leavy, ``Anything short of that... (and) the air campaign will continue.'' Still, the talks appear to be making progress, as Russian and U.S. military experts are taking part in the negotiating effort for the first time. Russia's envoy to NATO, Lt.-Gen. Viktor Zavarzin, who was recalled from Brussels at the start of NATO's bombing campaign against Yugoslavia, has also joined the talks. Before the extended trilateral session began in the afternoon, Chernomyrdin and Talbott met privately for two hours, once again going over the main stumbling block. Diplomats say it is now down to a matter of who will blink first -- NATO calling a temporary cease-fire or Milosevic beginning a large- scale, rather than a token, withdrawal of his forces from Kosovo. The other unresolved problem is NATO's position that it should form the core of a multinational armed force to enter Kosovo once the Serbs have withdrawn. Belgrade continues to insist that countries taking part in the current bombardment of Yugoslavia cannot take part in the force, but has reportedly agreed to a concession, accepting the participation of such NATO members as Portugal and Greece. These two countries have not been active in the alliance's military campaign in the Balkans. Chernomyrdin intends to fly to Belgrade for talks with Yugoslav President Slobodan Milosevic on Thursday, and hopes that Ahtisaari will join him. If the Finnish president leaves for Belgrade with Chernomyrdin, it will be a sign that a breakthrough in the negotiations is imminent. Meanwhile, the U.N. War Crimes Tribunal is preparing to indict Milosevic for command responsibility in the brutal ethnic purges of over 1 million ethnic Albanians from the Serbian province of Kosovo. NATO and Tribunal officials, five of whom agreed to discuss the case in recent days with UPI providing they were not identified, say the indictment is expected within two weeks.  