The US and Russia today agreed a nuclear disarmament road map that would see them cut their arsenals by up to a third, in a preliminary agreement signed by Barack Obama during his Russia trip.

Pledging to reverse a "sense of drift" in Washington's relations with Moscow, the US president said he hoped a new nuclear arms reduction treaty to replace the Start-1 pact, which expires this December, would be ready by the end of the year. "We must lead by example and that is what we are doing here today," he said in Moscow.

Under terms of the outline deal the sides have agreed to reduce their nuclear stockpiles to between 1,500-1,675 warheads each and that strategic delivery systems – ballistic missiles, submarine-launched missiles and heavy bombers – be cut to between 500 and 1,100.

But it was unclear today whether negotiations between the US and Russia would actually yield a new treaty – or whether both sides could bury their differences over the former's missile defence plans. The Kremlin has made it clear that a deal is impossible if the US administration goes ahead with its missile defence shield in Poland and the Czech Republic.

Obama said today that a review of that shield would be completed as early as this summer. But he gave no indication whether he was willing to dump it – instead merely predicting that the diametrically opposed positions of the two nations on the shield "could be reconciled".

Obama also insisted the purpose of the shield was to intercept missiles from Iran or North Korea or other states rather than from Russia. But he conceded that convincing Moscow of this would be hard work. "It's going to take time to break down existing suspicions," he noted.

Despite Obama's pledge to reset "US-Russian relations", there was little sense from today's summit that the two sides had managed to overcome the hostility and suspicion that characterised relations between George Bush and Vladimir Putin. Nor was there much of the sparkle that has accompanied previous summits between US and Russian leaders.

Asked whether he trusted Dmitry Medvedev, Obama responded by calling Russia's president "straightforward and professional". But he also had problems pronouncing his Russian counterpart's name – dubbing him on one occasion: "Mededev" – and appeared tired after the flight from Washington.

Analysts said the nuclear deal at the very least revived the notion of disarmament, which had been lost amid the hostilities of recent years, and was realistic.

"The negotiations are going to be tense," said Paul Ingram, the executive director of the British American Security Information Council. "The Russians will be playing hardball but the Americans know Moscow has a strong interest in getting a treaty signed. Both sides have too much invested in reaching an agreement."

Once the treaty is signed, the next question will be how much further the US and Russia have to go. Obama has dedicated himself to a world free of nuclear weapons, but that remains a theoretical target.

Hovering in the gilded rooms of the Kremlin like an unwelcome ghost was Putin, whom Obama meets tomorrow for a brief working breakfast. Asked whether he thought Putin or Medvedev ran Russia, Obama replied: "Medvedev is the president and Putin is the prime minister."

Russia did offer one significant concession, agreeing to let the US fly troops and munitions across its airspace to provide an air corridor for its forces in Afghanistan. The two sides also agreed to resume military co-operation, suspended following Russia's invasion of Georgia last year.

Obama reaffirmed US support for Georgia's "sovereignty and territorial integrity". There was no mention of Ukraine, whose admission to Nato Moscow ardently rejects along with that of Georgia's.