SYDNEY, May 23 (Reuters) - Papua New Guinea (PNG) will not be used as a base for "war to be launched", and a defence agreement with the United States prohibited "offensive military operations", its prime minister said on Tuesday.

U.S. Secretary of State Antony Blinken said on Monday a defence cooperation deal signed with PNG earlier that day would expand the Pacific island nation's capabilities and make it easier for the U.S. military to train with its forces.

The deal sparked student protests amid concern it could embroil PNG in strategic competition between the U.S. and China. read more

Prime Minister James Marape said the agreement was not a treaty and did not need to be ratified by parliament, adding he would release it in full for public scrutiny on Thursday.

"It's not a military base to be set up here for war to be launched," he told radio station 100FM.

"There's a specific clause that says that this partnership is not a partnership for PNG to be used as a place for launching offensive military operations from Papua New Guinea," he said.

The United States and its allies are seeking to deter Pacific island nations from building security ties with China, a rising concern amid tension over Taiwan, and after Beijing signed a security pact with the Solomon Islands last year.

China has been a major infrastructure investor in PNG, which sits near important sea lanes and international submarine cables linking the United States and ally Australia, that were crucial in World War Two.

Marape said PNG's military is the weakest in the region at a time of high tensions. The boost provided by the United States would also improve domestic security and encourage more foreign investors to set up in the country of 9 million that is rich in natural resources but largely undeveloped.

"There will be substantial infrastructure investment" flowing from the deal, in airports, ports, roads, communications and electricity to benefit the public, he said, while not giving details.

Subsidiary agreements that will determine how the U.S. military and civilian contractors come into PNG are being worked out, he said.

Australia has welcomed the defence cooperation agreement between its closest neighbour and the United States.