ONBOARD A MILITARY AIRCRAFT - The United States is reaching out to allies in response to Russia's military build-up along its border with Ukraine.

U.S. Defense Secretary Lloyd Austin spoke by phone with Polish Minister of National Defense Mariusz Blaszczak Tuesday to discuss what the Pentagon has repeatedly described as Moscow's "unusual military activity."

The two also shared concerns about the situation along Poland's border with Belarus, where Polish officials have accused the Belarus regime of using thousands of migrants to raise tensions.

They "discussed ways to enhance deterrence along NATO's Eastern Flank," according to a U.S. Defense Department readout of the call.

Austin made the call while en route to Seoul for talks with his South Korean counterpart.

Ukrainian intelligence officials say Russia has positioned about 90,000 troops along the border with Ukraine and in Crimea, which it seized from Ukraine in 2014.

Multiple U.S. officials have said Washington and its European allies are planning for "a number of contingencies," fearing Moscow's saber-rattling may be more than tough talk.

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U.S. officials have called U.S. support for Ukraine "rock solid" and are considering a number of tools to deter Moscow.

Separately, a top British intelligence official warned Tuesday that Moscow is engaging in increasingly "brazen activity."

'We and our allies and partners must stand up to and deter Russian activity which contravenes the rules-based international system," MI6 Chief Richard Moore said during a speech at the International Institute of Strategic Studies.

'Moscow should be in no doubt of our support for the sovereignty and territorial integrity of Ukraine, within its internationally-recognized borders, including Crimea," Moore added.

Russian officials, for their part, have accused the U.S. and its allies of stoking tensions in the region.

Russia's TASS news agency quoted Russian Defense Minister Sergey Shoigu last week as saying Moscow was "witnessing a considerable increase in the U.S. strategic bombers' activity near the Russian borders."

Shoigu also alleged U.S. strategic bombers "practiced employing nuclear weapons against Russia actually simultaneously from the western and eastern directions.'

The U.S. has provided Ukraine with $2.5 billion in security assistance since 2014, including $400 million in 2021 alone.

Recent deliveries include patrol boats for the Ukrainian navy and 80,000 kilograms of ammunition for Ukrainian forces.