NATO risks being irrelevant unless more money is pumped into its forces, said US defence secretary Robert Gates.

In a sharp parting shot to his European allies before retiring at the end of the month, Gates said NATO-led operations in Afghanistan and Libya had exposed significant shortcomings in military capabilities and political will among the allies.

The United States’ £862.4 billion deficit may bring its 75 per cent share of NATO defence spending into question, and pose”a real possibility for a dim, if not dismal future for the trans-atlantic alliance,” Gates said.

“The blunt reality is that there will be dwindling (U.S.) appetite and patience … to expend increasingly precious funds on behalf of nations that are apparently unwilling to devote the necessary resources or make the necessary changes to be serious and capable partners in their own defence,” Gates said.

“If current trends in the decline of European defence capabilities are not halted and reversed, future U.S. political leaders — those for whom the Cold War was not the formative experience that it was for me — may not consider the return on America’s investment in NATO worth the cost.”

Despite having more than 2 million troops in uniform, non-U.S. NATO states struggled to sustain 25,000 to 45,000 troops in Afghanistan, “not just in boots on the ground, but in crucial support assets such as helicopters, transport aircraft, maintenance, intelligence, surveillance and reconnaissance.”

Gates said the air operations against the forces of Libyan leader Muammar Gaddafi had further exposed NATO’s limitations, with an air operations centre designed to handle more than 300 sorties a day struggling to launch about 150 and the United States having to make up shortages of munitions.

NATO had to change its approach, he said.

“My hope is that the fact that the reality is changing in the United States will get the attention of European leaders to realise that the drift of the last 20 years can’t continue, not if they want to have a strong trans-Atlantic partnership.”

Gates’s remarks followed two days of NATO meetings at which he said too few nations were bearing the bulk of the burden in Libya, and singled out five that he urged to do more.

Officials said he asked Spain, Turkey and the Netherlands to fly strike missions in addition to the air operations they currently undertake. He urged Germany and Poland, which are not contributing, to find ways to help, the officials said.

Gates singled out Norway as a small country punching above its weight by joining air strikes in Libya, but Oslo said on Friday its mission would end on August 1 and it would reduce its contribution of six F-16 fighters to four on June 24.

“Our allies should understand that Norway’s small air force cannot sustain a great effort over a long period of time,” Defence Minister Grete Faremo said in a statement.

NATO spokeswoman Oana Lungescu said alliance Secretary-General Anders Fogh Rasmussen shared Gates’s concerns.

“There is clearly a long-standing concern about the trans-atlantic gap in defence spending. There is a risk that European allies may fall even further behind in terms of technological developments,” she said.

Former NATO Secretary-General Jaap de Hoop Scheffer warned the forum the current imbalance was “not sustainable.”

“Europe has a rather pale face as we speak,” he said, criticising “totally uncoordinated budget cuts” and urging his own nation, the Netherlands, to join strike missions in Libya.

Obama has nominated outgoing CIA chief Leon Panetta to take over from Gates at the Pentagon.