U.S. President Barack Obama meets with Russia's President Dmitry Medvedev at Winfield House in London on Wednesday. (Charles Dharapak/Associated Press)

The United States and Russia committed Wednesday to resetting strained relations, agreeing to quickly negotiate a new treaty to limit nuclear weapons.

The meeting in London was the first face-to-face meeting between presidents Barack Obama and Dmitry Medvedev. Both leaders are in Britain preparing for the start of G20 talks on Thursday.

After their first meeting, the White House also announced that Obama was accepting Medvedev's invitation to visit Moscow this summer.

Both leaders declared in their joint statements that the "era when our countries viewed each other as enemies is long over."

While agreeing to work overtime to negotiate a replacement for the seminal 1991 Strategic Arms Reduction Treaty, or START, which expires at year's end, the two leaders vowed at the same time to jointly confront other perceived threats.

Both men directed their negotiating teams to finish the task of setting broad outlines for a treaty to replace START by the end of July.

"The parties have gone from words to action," said Sergei Rogov, the head of USA and Canada Institute.

Currently, the United States has 2,200 strategic nuclear warheads deployed; Russia has 2,800.

New president urges leading role by U.S., Russia

Obama has declared his belief that the United States and Russia should take the lead in ridding the world of nuclear weapons altogether.

Russian and American arms control experts believe that the START replacement treaty would seek initially to cut strategic warhead arsenals to 1,500 on each side.

"Over the last several years, the relationship between our two countries has been allowed to drift," Obama told reporters after his meeting with Medvedev.

"What I believe we've begun today is a very constructive dialogue that will allow us to work on issues of mutual interest."

Striking a similar tone with the U.S. president at his side, the Russian president said: "I am more optimistic of the successful development of our relations."