Russia and the United States began Monday a new round of negotiations on renewing a key nuclear arms reduction treaty, just days after Washington said it would drop missile shield plans in Europe.

"The negotiations have restarted just after 1100 local time (0900 GMT) in the Russian mission. This time, it would last longer than expected with the large delegations from each side," a Russian diplomat told AFP.

The six previous rounds of talks on the renewal of the Strategic Arms Reduction Treaty (START) which expires on December 5 had lasted a maximum three to four days.

The United States and Russia had agreed this year to seek the replacement or renewal of START, marking the first tangible step in the thaw in US-Russian relations heralded by the Obama administration.

START, signed in 1991 just before the break-up of the Soviet Union, bound both sides to deep cuts in their nuclear arsenals.

Negotiations have been dogged by bargaining over the deployment of the US missile defence shield in ex-Soviet states in eastern Europe, a project that has angered Russia.

But US President Barack Obama last week announced that he would shelve plans to site parts of a missile defence shield in Poland and the Czech Republic, and instead deploy more mobile equipment targeting Iran's short and medium-range missiles.

The White House has denied it orchestrated a quid pro quo with Moscow by agreeing to halt work on the missile shield in a bid to win more cooperation on issues like Iran's nuclear programme.

Obama and Russian President Dmitry Medvedev are set to meet later this week when the Russian leader travels to New York to address the United Nations.

However, Medvedev's foreign policy advisor Sergei Prikhodko had said it would be unreasonable to expect a quick agreement on the START nuclear arms control treaty or any other deals when the presidents meet face to face.

"Contradictions remain," Prikhodko added.