WASHINGTON - U.S. negotiators working to conclude a new strategic arms treaty with Russia are discussing ways to continue nuclear weapons monitoring until the new accord can be ratified, a State Department spokesman said on Monday.

U.S. President Barack Obama and Russian President Dmitry Medvedev agreed in July to work on a new treaty that would cut their deployed strategic nuclear arsenals to between 1,500 and 1,675 warheads.

The current Strategic Arms Reduction Treaty between Russia and the United States expires on December 5. Negotiators in Geneva are hopeful of reaching a draft agreement around that time, but the deal would still have to be ratified by both sides, State Department spokesman Ian Kelly told reporters.

The negotiating teams continue to work very hard in Geneva. They have agreement on a number of issues but they are also trying to work out some of the areas where they need to come together, he said.

Because the treaty has to be ratified by the respective legislatures, we ... know that we are not going to have a ratified treaty that can enter into force, Kelly added. So we are having discussions with Russia to see how we can continue some of the transparency and verification measures ... until the treaty is ratified.

Senator Richard Lugar, the ranking Republican on the Foreign Relations Committee, introduced a bill last week that would permit Russian arms experts to come to the United States to carry out inspections permitted under the START treaty.

The measure would let Obama approve the inspections as long as the Russians extended similar permission to U.S. arms experts.

Lugar, in a Senate speech, said extending the START verification mechanism was particularly important because it is also used to monitor the 2002 Moscow Treaty on strategic nuclear forces.

Kelly said the Lugar legislation was part of the effort to extend the START weapons inspection and verification regime.

Since we recognize we're not going to have a fully ratified treaty in both capitals, we're looking at ways that a number of provisions can remain in effect in this period between December 5 and whenever the new treaty is ratified, he said.

These monitoring mechanisms are important, Kelly said. You need to have some kind of mechanism to keep these means of monitoring in place and ongoing.

Estimates of current nuclear stockpiles vary, but the U.S.-based Bulletin of the Atomic Scientists estimated at the start of 2009 the United States had about 2,200 operationally deployed nuclear warheads and Russia had about 2,790.

(Editing by Cynthia Osterman)