Speaking of a 2003 agreement governing the Kerch Strait between the Azov Sea and Black Sea, Mattis said Moscow had shown "brazen contempt and dismissal" for the deal "that allowed both Russian and Ukrainian ships free passage."

Mattis was speaking before an audience at a defense forum in California, a day after President Donald Trump went out of his way to dodge Russian leader Vladimir Putin at the G20 summit in Buenos Aires, citing the capture of the Ukrainian ships.

The defense chief's remarks represent some of the strongest reaction to date by the US over the incident.

Last week, US Ambassador Nikki Haley to the UN accused Russia of "outlaw actions" and an "outrageous violation of sovereign Ukrainian territory" while addressing an emergency meeting of the Security Council.

Tensions between Ukraine and Russia spiked on November 25 when Russian forces opened fire on and seized three Ukrainian navy vessels, detaining the 24 crew members.

It was the first open military confrontation between the rivals since 2014, when Russia annexed Ukraine's Crimea peninsula and pro-Russian separatists in the east entered into conflict with Ukrainian forces.

Mattis also blamed Russian President Vladimir Putin for President Donald Trump's decision in October to withdraw from the 1987 Intermediate-Range Nuclear Forces Treaty (INF).

"We are dealing with Putin's duplicitous violation of the INF treaty," he said, noting that while the US remains in compliance, Russia does not.

"We will re-energize our arms control efforts, but the onus is on Russia," he said.