The US military faces serious budget cuts and will be unable to make up any shortfalls in the NATO alliance as European members slash defence spending, Pentagon chief Leon Panetta warned Wednesday.

Fiscal pressures are bearing down on both sides of the Atlantic and NATO allies will need to work closely together to pool funds, instead of counting on America's much larger defence spending to close the gap, Panetta said.

"As for the United States, many might assume that the US defence budget is so large it can absorb the shocks and cover alliance shortcomings -- but make no mistake, we are facing dramatic cuts with real implications for alliance capability," the US defence secretary said in a speech in Brussels.

Although US defence spending far exceeds European budgets, Panetta said American military leaders were facing $450 billion in cuts over 10 years, which he called "tough but manageable," according to a prepared text of the speech.

But if the US Congress fails to tackle the country's deficit this year, the Pentagon "could face additional cuts in defence that would be devastating to our national security and to yours as well," he said.

Panetta delivered his warning ahead of talks with NATO counterparts in Brussels, centred on the missions in Libya and Afghanistan as well as the shortcomings the alliance has witnessed in carrying out the operations.

The cuts contemplated by the Pentagon would reduce the size of the force and curtail some weapons programmes, but the gargantuan US defence budget -- at nearly $700 billion -- still dwarfs that of the 27 other NATO members combined.

US officials have long urged European allies to shoulder more of the burden of the alliance. But Panetta stressed that a new era of austerity would require member states to coordinate budget cuts to minimize the impact on NATO's military might.

"We cannot afford for countries to make decisions about force reductions in a vacuum, leaving neighbors and allies in the dark," Panetta said at an event organised by the think tank Carnegie Europe.

"Security in the 21st century will not be achieved by each nation marching to its own drummer," he said in his first speech in Europe since taking over as defence secretary in July.

Panetta struck a gentler tone than his predecessor, Robert Gates, who delivered a harsh rebuke to the alliance in June before his retirement.

While Gates painted a bleak picture of an alliance on the verge of "irrelevance" after failing to invest in defence, Panetta praised NATO for its "extraordinary" performances in Libya and Afghanistan.

"With the fall of the Kadhafi regime, our nations saw an example of why NATO matters, and remains indispensable to confronting the security challenges of today," Panetta said.

In the Libya air war, NATO proved it could make move swiftly, effectively and with Europeans -- instead of Americans -- playing the lead role in a major operation, he said.

"The alliance achieved more burden-sharing between the US and Europe than we have in the past, particularly for an operation conducted off of Europe's shores," he said.

Echoing US and European officials, Panetta said the Libyan and Afghan conflicts exposed worrisome gaps in alliance capabilities, including a shortage of drones, refuelling tanker aircraft, helicopters, munitions and targeting specialists.

As an example of how the alliance needs to share resources, Panetta cited a proposal for 13 countries to invest in several unmanned surveillance aircraft, calling it a "true bargain for NATO."

But he said the programme to build Global Hawks was stalled due to disagreements about funding, and argued the intiative offered a test of the alliance to chart a new path with limited funds.

Panetta said he believed the alliance would endure despite the financial squeeze.

"It has been a tough decade of war, but our alliance has emerged stronger. Just as we met the challenges of the Cold War and 9/11, I am confident we can confront the challenges that await us in the next decade."