WASHINGTON (Reuters)-Mediators hope to clinch an agreement between Azerbaijan and Armenia on the principles of a peace deal on Nagorno-Karabakh at talks tentatively planned for mid-July in Russia, the U.S. negotiator said over the weekend.

U.S. Deputy Assistant Secretary of State Matthew Bryza said he and his fellow mediators from France and Russia were "shooting for" a full framework agreement by the end of 2009. But he conceded the risk of a last-minute breakdown of the kind that derailed earlier efforts to broker agreement between the Caucasus neighbors, who continue to exchange fire over their tense frontline 15 years after major hostilities ended.

"I don't have any reason necessarily to believe that getting as far as we have here - which is similar to how far the mediators and the parties got both at Key West and before - that we're going to get further than they did," he told Reuters by phone from Washington. "I do know that we've gotten very far now. What gives me some hope that we will keep moving is logic."

Bryza said the Minsk Group mediators of the OSCE hoped to bring together Armenian President Serzh Sarkisian and Ilham Aliyev of Azerbaijan in mid-July, the latest in a string of encounters fuelling speculation of a breakthrough.

"We hope that if they meet in the middle of July, they will have agreed conceptually on all the elements of these basic principles," he said. The parties would then go line by-line through the three-and-a-half pages of text to agree the details.

"Once that happens, which we the co-chairs are shooting for by the end of the year, then we could say, it would be true, that a framework agreement has been reached," he said.

Armenia supports Nagorno-Karabakh's right to self determination and independence, something Azerbaijan rejects and has said it is ready to go to war to prevent. Bryza said the mediators were bridging the gap, but that the final deal would likely provide for a vote "that reflects the genuine will of the populations".

"What we are trying to do is incorporate self-determination through a voting process on Nagorno-Karabakh's final legal status, but in a way that for the foreseeable future has no impact on Azerbaijan's territorial integrity." He declined to elaborate.

Analysts say a deal would first require the withdrawal of Karabakh defense forces from seven districts surrounding the Nagorno-Karabakh Republic, get international peacekeepers in place, and tackle the issue of hundreds of thousands of Azeri refugees. Bryza said Russia's war with Georgia last August, when Moscow crushed a Georgian assault on the breakaway region of South Ossetia, had "sharpened the minds" of Sarkisian and Aliyev.

Analysts also point to a thaw between Azeri ally Turkey and Armenia, bringing landlocked Armenia tantalizingly close to seeing its border with Turkey reopened 16 years after Ankara closed it in support of Azerbaijan during the war.

Turkey and Armenia announced a roadmap to normalize ties in April. But Turkey has since said it will not open the frontier until Armenia makes concessions on the Nagorno-Karabakh conflict to Azerbaijan's satisfaction.

Bryza, also closely involved in the Turkey-Armenia roadmap, said the processes were separate, but running in parallel. Asked if Turkey would only reopen the border once Armenia makes concessions on Nagorno-Karabakh, he replied: "I do not know if that's right," but added: "Where there is unanimity, is that we all say we need to see a breakthrough on Nagorno-Karabakh and significant progress as soon as possible, and that's the way to make sure all these processes move forward smoothly."