A Soyuz capsule carrying three astronauts has successfully docked with the International Space Station, bringing the size of the crew at the orbiting outpost to six.

Michael Hopkins of the United States and Russians Oleg Kotov and Sergei Ryazansky traveled six hours in the capsule from the Russian-leased launch pad in Kazakhstan before linking up with the space station's Russian Poisk research module at 6.45am Moscow time (02.45am GMT).

Kotov is the most experienced member of the crew with two six-month missions in space under his belt while Hopkins and Ryazansky are both on their first missions.

The incoming crew entered the station nearly two hours later where they were welcomed by Russia's Fyodor Yurchikhin, Nasa's Karen Nyberg and Italy's Luca Parmitano.

The crew's six-month mission will include a spacewalk with the Olympic torch.

Kotov and Ryazansky will have the honour of taking the Olympic torch into open space in November as part of the relay of the Olympic flame ahead of the Winter Games being held in Russia's Sochi in February. The torch will not be lit however, because of safety concerns and will only arrive at the station in November with the next mission.

Shortly after entering the station Hopkins, Kotov and Ryazansky had a chat via a video-link with their families back at the Baikonur launch pad who saw them off more than eight hours before.

Radiant but visibly tired astronauts were yawning as they were talking to their families. They have been up for about 20 hours.

Hopkins' mother described the launch as a "heart-stopping experience".

"It was a pretty good ride, mom. It was a lot of fun," Hopkins replied in the live broadcast on Nasa TV.