SINGAPORE -- President Obama and Russian President Dmitry Medvedev said tough issues divide the two in on-going nuclear arms reduction talks and aides now openly acknowledge both nations will need a "bridge" agreement because a new deal won't be hammered out before the current treaty expires Dec. 5.

"Our goal continues to be to complete the negotiations and be able to sign a deal before the end of the year," Obama said. "And I am confident that if we work hard and with a sense of urgency about it, that we should be able to get that done. Both sides are trying to work through some difficult technical issues but are doing so in good faith."

Obama's highly conditioned assessment of the future of talks with Russia underscored increasing American anxiety over the pace of negotiations.

The Strategic Arms Reduction Treaty, or START, requires both nations to reduce stockpiles of nuclear warheads to 6,000 and 1,600 delivery systems -- missiles fired from land, sea or air. The goal of the new START deal is to reduce warheads to 1,500-1,600 and delivery systems to between 500 and 1,000.

Sticking points include how to inspect and verify destruction to warheads and delivery systems and which system and warheads count as genuine reductions. The Russians accuse the U.S. of seeking to count the destruction of obsolete warheads and missiles against the number of usable weapons.

The Russians, generally, are hesitant to open their bases and facilities to intrusive U.S. verification inspections.

"We've agreed to give additional impetus to those (new START) negotiations, find solutions on remaining issues," Medvedev said. "In some instances, those are technical issues, some are political issues. We will task our aides to continue working on those matters. I hope that...we will be able to finalize he text of a document by December. The world is watching. It is all the more important now."

Mike McFaul, the president's top National Security Council adviser on Russia, acknowledged for the first time both nations are now negotiating a second treaty, a so-called "bridge" pact that would maintain the current START treaty while talks continue on its successor. To finish a bridge deal, both sides have to agree to current intrusive verification methods, a key test of current trust between Medvedev and Obama.

"Today, I would describe it as progress," McFaul said of the Obama-Medvedev talks. "We're not at the end game yet. We still have some fairly major things to finish. To get into the details....we're just not going to talk about the negotiations day-by-day. We said we're going to sign a treaty by the end of the year.”

McFaul later said Obama and Medvedev resolved some areas of contention, but declined to describe them or their relative importance in shaping a final deal.

"We made progress and I really don't want to talk about the specifics, because the Russians have asked us not to talk about the specifics."

When asked if there would be a summit surrounding the signing of the new treaty, McFaul demurred.

"They (Obama and Medvedev) said we're going to get it done by the end of the year. We have not talked about the modalities of when or when or how," McFaul said. "We just haven't gotten to that point."

McFaul also said the "bridging agreement hasn’t been negotiated. What it will say, we'll have to wait until we get an agreement. It's more of a technical issue. But I need to be clear, we don't have an agreement yet. It would be to preserve the verification."

When asked how long this underlying bridge deal would last, McFaul said: "It will depend, a lot, on when we get the new START agreement, both those things will go together. We don't see any gaps down the road in terms of a bridging agreement, a bridge leads to somewhere, it leads to the other agreement."