KIEV (AFP) - The United States would support greater autonomy for Crimea as long as this does not happen "under the barrel of a gun", the US ambassador in Kiev said on Monday.

"The United States believes that Crimea is and should remain a part of Ukraine," Geoffrey Pyatt told reporters in Kiev, stressing that Washington would not recognise a referendum being planned there by pro-Russian lawmakers on Sunday.

But he also recognised Russia had "legitimate interests" on the peninsula, where its Black Sea Fleet is stationed.

"I have been encouraged to see comments from prime minister (Arseniy) Yatsenyuk ... indicating the Ukrainian government is open to enhanced levels of autonomy for the people of Crimea," he said.

"But this is something that Ukrainians have to decide within their own constitutional framework and it can't happen under the barrel of a gun," he said.

The March 16 referendum, which has been deemed illegitimate by the new government in Kiev, will ask Crimeans if they want their region to join Russia or if they prefer greater autonomy within Ukraine.