BRUSSELS/WASHINGTON (Reuters) - The United States is pressing its European allies to get more NATO battalions, ships and planes ready for combat, officials say, in a fresh move to shore up NATO’s deterrence against any Russian attack.

U.S. Secretary of Defence Jim Mattis will seek broad agreement for the plan in Brussels on Thursday when alliance defence ministers meet, laying the ground for endorsement by NATO leaders at a summit in July, four U.S. and NATO officials and diplomats told Reuters.

Known as 30-30-30-30, the plan would require NATO to have 30 land battalions, 30 air fighter squadrons and 30 ships ready to deploy within 30 days of being put on alert.

It does not discuss specific troop numbers or a deadline for setting up the strategy. The size of battalions vary across NATO, from 600 to 1,000 soldiers.

That lays down a challenge for European governments, pilloried by U.S. President Donald Trump for slashing military spending after the Cold War, to remedy long-running problems with helicopters and jets that are grounded for lack of parts.

“We have an adversary (Russia) that can move quickly into the Baltics and Poland in a ground attack,” said one senior NATO diplomat who was briefed on the U.S. plans. “We don’t have the luxury of taking months to mobilise.”

One U.S. official said the initiative was primarily aimed at countering Russia and fit with the Pentagon’s 2018 National Defence Strategy, which accuses Moscow of seeking to “shatter the North Atlantic Treaty Organisation.”

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Russia’s war games last year, involving what Western officials said were 100,000 troops, also prompted concerns about accidental conflicts that could be triggered by such exercises, or any incursions into Russian-speaking regions in the Baltics.

The Kremlin firmly rejects any such aims and says NATO is the security threat in eastern Europe.

“This idea, even if it flies, which I hope it won’t, would only increase tensions in an increasingly sensitive part of Europe,” Russia’s envoy to the European Union, Vladimir Chizhov, told reporters when asked about the proposal.

Wrong-footed by Moscow with Russia’s seizure of Crimea in 2014 and its intervention in Syria’s war in 2015, the United States is distrustful of the Kremlin’s public message and wants to be ready for any eventuality.

WHOSE TROOPS?

With more than 2 million troops, NATO forces outnumber Russia’s, whose active duty members are around 830,000, according to the International Institute of Strategic Studies, a British-based military think-tank.

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Russia’s annexation of Crimea prompted NATO to set up a small, rapid-reaction “spearhead” force and put four battalions in the Baltics and Poland, backed up by U.S. troops and equipment on rotation.

But is unclear how fast the alliance could move large troop numbers to its eastern flank and how long it could sustain them. France is already stretched thin in Africa and British cutbacks are reducing the size of deployable forces, officials said.

According to a 2016 study by the Rand Corporation, Britain, France, and Germany could each muster a brigade of three or more battalions along with battle tanks and other armour in around a month. But their resources would be badly strained, leaving little capacity for any other conflicts.

Another unknown is how the 30-30-30-30 proposal would fit in with other initiatives aimed at improving European combat readiness and addressing shortfalls in weaponry and other military assets.

The EU last December formed a defence pact to develop crisis-response forces and work together to develop new helicopters and ships. French President Emmanuel Macron aims to set up a new French-led European “intervention force”.

“We only have a certain amount of forces in Europe, and they cannot be committed to every military proposal,” a second senior NATO diplomat said.