Defence Secretary Penny Mordaunt said the British-led Joint Expeditionary Force, a rapid response unit, has reached a "new scale" in the exercise that included nearly 4,000 people and 44 vessels from nine nations.

Mordaunt called the exercise "the largest Royal Navy deployment in the Baltic for more than a hundred years", speaking to reporters aboard a British warship in the Lithuanian port of Klaipeda, which is just a stone's throw from the militarised Russian exclave of Kaliningrad.

The exercise was meant to reassure eastern flank NATO allies and to demonstrate that they are "aligned and ready".

"Russia is becoming more assertive, we see her deploying more forces and new weapons, and we can imagine scenarios that may play out in the future," Mordaunt said.

"So it is important and right that we stand together with our allies."

In a separate interview with the Baltic News Service, Mordaunt said Britain wanted "to do more" to "build security and resilience in the Baltic states and the region", including exercises and regular patrols, but declined to elaborate.

The British defence secretary's visit to Lithuania, a vocal critic of Russia, came on the same day as outgoing British Prime Minister Theresa May met Russian leader Vladimir Putin on the sidelines of the G20 summit in Japan.

May told Putin that normal relations will not be restored until Moscow ends its "irresponsible and destabilising" activity, according to a Downing Street spokesperson.

Alongside Lithuania, another seven northern European nations are part of the Joint Expeditionary Force, including fellow Baltic states Latvia and Estonia and non-NATO member Sweden.