(Adds Obama on research to continue)

By Matt Spetalnick and Michael Winfrey

PRAGUE, April 5 (Reuters) - The United States will develop anti-missile defences as long as an Iranian nuclear threat persists, and North Korea must be made to change after its rocket launch, U.S. President Barack Obama said on Sunday.

Obama's administration had been cool on a deal reached by his predecessor, George W. Bush, to put radar in the Czech Republic and interceptor rockets in Poland to shoot down missiles fired by countries like Iran or North Korea.

But the issue grabbed the spotlight on Sunday after North Korea launched a long-range missile, drawing condemnation from the international community although Pyongyang said it had only put a satellite in orbit.

During a visit to the Czech capital on Sunday, Obama said Washington would continue with the plan as long as Iran was developing its nuclear programme.

"As long as the threat from Iran persists, we will go forward with the missile system," Obama said in a speech.

"If the Iranian threat is eliminated, we will have a stronger basis for security, and the driving force for missile construction in Europe will be removed."

Earlier Gary Samore, White House coordinator for arms control, said the North Korean launch meant missile defence would remain a priority.