Pompeo is in Greece to underline support for a NATO ally and reinforce cooperation, signing an amendment to a mutual defence cooperation agreement, sources for both governments said.

"This is a dynamic region, with lots going on, lots of change taking place and we are very confident that together, we can work to ensure that Greece can be a pillar for stability in this region," said Pompeo, hailing a relationship that "has truly never been stronger."

Thousands of demonstrators outside the Greek parliament begged to differ, some brandishing a huge banner reading "Pompeo go home."

His visit comes amid Greek concerns to which Prime Minister Kyriakos Mitsotakis alluded -- Turkish drilling off EU member Cyprus.

The discovery of huge gas reserves in the eastern Mediterranean has triggered a race to tap the region's resources and sparked a dispute between Turkey and Cyprus.

"The United States has a special interest in the Eastern Mediterranean region. Cyprus only asks for the obvious, the implementation of international law.

"I expect the positive contribution of the United States, which will eventually lead to the creation of a more constructive and productive environment in the region," Mitsotakis added.

Turkey has had troops stationed in Cyprus since 1974 when it invaded and occupied its northern third after a coup sponsored by the military junta then ruling Greece.

As well as holding talks with Mitsotakis, who took office in July, Pompeo was also meeting Greek counterpart Nikos Dendias and Defence Minister Nikos Panagiotopoulos.

Pompeo refused to address questions from reporters on allegations that President Donald Trump pressured Ukraine for political favours, a claim which has launched an impeachment investigation.