* Premier, Rockhopper partner to develop oil off Falklands
* Argentina says activities illegal, vows legal action
BUENOS AIRES, July 16 (Reuters) - Argentina will take legal action against Britain's Premier Oil after the company agreed to invest $1 billion to help develop an oil field off the disputed Falkland Islands, Argentina's Foreign Ministry said on Monday.
Argentina and Britain fought a war 30 years ago over the UK-governed islands, which are known as the Malvinas in Spanish. Argentina claims the Falklands as its own and has sought to disrupt drilling there.
Last week, Premier said it would partner with Rockhopper , an explorer that discovered oil north of the Falklands in 2010, paying an initial $231 million in cash and providing around $770 million to help build infrastructure to secure a 60 percent stake in the Sea Lion field.
Argentina told Premier it had taken legal action against Rockhopper's "illegal" oil exploration activities on the "Argentine continental shelf."
"Likewise, the Argentine government informed Premier Oil that the government ... will initiate administrative, civil and penal action against it with regard to these activities," a statement by the Foreign Ministry said.
In March, Argentina vowed to take oil firms active in the Falklands -- and other companies providing support and services to them -- to the local courts for what it sees as illicit drilling in disputed territory.
Last week, Premier's finance director, Tony Durrant, played down the risks.
"I don't want to sound flippant, but the oil industry deals with this type of political risk, of border disputes, of disputed territories all around the world, frankly," Durrant told Reuters.