The US on Wednesday said it planned to conduct military training with Cyprus for the first time, defying Nato ally Turkey, which warned the move could threaten stability.
The US Congress last year ended a decades-long arms embargo on the island, the northern third of which is occupied by Turkey.
Secretary of State Mike Pompeo said the State Department would fund military training for Cyprus as part of "our expanding security relationship".
"This is part of our efforts to enhance relationships with key regional partners to promote stability in the eastern Mediterranean," Mr Pompeo said.
The co-operation will be part of the US International Military Education and Training programme, which trains foreign officers and seeks to increase co-operation with the US forces.
Ankara and the "Turkish Republic of Northern Cyprus" warned against the end of the Cyprus arms embargo, saying it would upset a balance of power on the island, and warned against the US training plan.
"As we repeatedly stressed in the past, these kind of steps do not contribute to efforts of finding a solution to the Cyprus problem, but further strengthen the Greek Cypriot side's uncompromising attitude," the Turkish Foreign Ministry said.
"It is clear that steps that do not observe the balance between the two sides will not help in restoring an environment of trust on the island, as well as restoring peace and stability in the eastern Mediterranean."
The US imposed the embargo in 1987 to avoid an arms race and encourage a peaceful resolution on the island, the population of which is mainly Greek.
Critics say the decision backfired by forcing Cyprus, now an EU member, to seek other partners.
At the same time Turkey, despite its Nato membership, angered the US by buying an advanced missile-defence system from Russia.
Turkey invaded Cyprus in 1974 after a coup engineered by the then military regime in Athens, which sought to unite the island with Greece.
The island has since remained largely peaceful, with Greek Cypriot and Turkish Cypriot leaders working to build ties in recent years.
But tension has recently flared over Turkey's drilling for gas off the island, with the EU calling the move illegal.
