U.S. President Barack Obama and his Russian counterpart Dmitri Medvedev are expected to review progress later this month on a follow-on treaty to replace the Strategic Arms Reduction Treaty, or START I, that expires this December.

During a July summit meeting in Moscow, Presidents Obama and Medvedev agreed on the basic principles of a treaty to replace the existing START I accord that expires December 5.

Complex accord

More than 1,000 pages long, the START agreement is one of the most complex treaties in history dealing with reducing nuclear weapons. It was signed in 1991 by U.S. President George Herbert Walker Bush and Soviet President Mikhail Gorbachev. It came into force in 1994.

Experts say the United States has about 2,200 strategic nuclear warheads deployed on approximately 1,000 delivery systems - land-based or sea-based missiles and heavy bombers. Russia has approximately 2,700 strategic nuclear warheads deployed on about 700 delivery systems.  

New parameters

Steve Andreasen, an arms control expert at the University of Minnesota, says at their July summit, Presidents Obama and Medvedev sketched out the parameters of a new accord to replace START I.

"They agreed that in terms of strategic nuclear warheads to be limited, the two sides would basically work to get to a range of 1,500 to 1,675 warheads on both sides. And they also agreed that on the question of limiting nuclear delivery vehicles, they would agree to limitations in a range between 500 and 1,100," he said.

The START I treaty also established stringent and very intrusive verification procedures.

Frank Miller, former senior official on the National Security staff under President George W. Bush, says those provisions were a crucial part of the treaty.

"They were extremely important, certainly to [U.S. Senate] ratification," he said. "They overcame a long period of distrust between the United States and the then Soviet Union because it allowed each side to have greater confidence that the other side was abiding by the rules in the treaty."

Verification is key

Many experts, including Daryl Kimball, executive director of the Arms Control Association, a private research firm, say one of the key issues facing current U.S. and Russian negotiators is verification.

"How many of the verification and monitoring provisions from the existing START agreement will be carried over in the future. Those provisions were negotiated during a time when there was much less trust and transparency between the two countries. Today, both sides agree that fewer inspections are needed, fewer monitoring techniques are needed - but they still need to agree on which ones," he said.

Experts also say negotiators have to agree on so-called "counting rules" - what strategic nuclear delivery systems will be counted in the new accord and how many warheads will be attributed to those delivery vehicles.  

"The Russians want to count delivery systems. The Americans  would rather just count the warheads. There will be some compromise in the end," said Daryl Kimball.

Will deadline be met?

Arms control negotiators are racing to get the treaty completed by the December 5 deadline. To enter into force, the new pact will have to be ratified by the Russian parliament - or Duma - and the U.S. Senate.

Former senior National Security official Frank Miller says it is doubtful the ratification deadlines will be met.

"I have talked with people on the staff of the Senate Foreign Relations committee and they suggest that it would take several months for the Senate to organize itself and to hold proper hearings, which would allow U.S. Senate advice and consent to the treaty, ratification of the treaty. So we do face a prospect that the START treaty will expire without a replacement actually being in place by December 5th," he said.

Miller looks at some alternatives available to negotiators.

"Well, if the parties are negotiating in good faith, they can agree that they will continue the existing treaty in force, in a de facto if not de jure manner, until the new treaty is in place," he explained. "They can agree not to take any steps which would undercut the START I treaty and the follow-on treaty that is being negotiated, until such time as the follow-on treaty is ratified and in place."

Recent statements by American and Russian officials indicate progress has been made since Presidents Obama and Medvedev in July gave their negotiators the parameters of a START I follow-on treaty.

But experts say it is unclear whether the progress is enough to complete a new strategic arms control pact by the end of the year.