A Russian cargo rocket has arrived at the International Space Station, but is having problems docking securely.

An unmanned Russian Progress supply spacecraft has failed to latch completely to the station's Russian Zvezda module after an automated docking.

A spokesman for U.S. space agency mission control in Houston, John Ira Petty, says the problem is a stuck antenna on the supply craft.

"It's believed that that antenna has not been fully retracted and until flight controllers can confirm that that retraction has taken place or send commands that will complete that retraction, the Progress 23 remains captive at the aft end of the Zvezda Service Module, but the driving of hooks and latches, which will firmly secure it to that docking port, has been delayed," he said.

The problem is preventing a proper seal between the cargo rocket and the station, delaying the unloading of the 2.5 tons of supplies. But space officials say the U.S., Russian and German crewmembers are not in danger.