NATO must rethink its long-term relationship with Russia and strengthen the alliance's ability to respond quickly to any threat, British Prime Minister David Cameron said Monday.

Russia will ban all imports of food from the United States and all fruit and vegetables from Europe, the state news agency reported on Wednesday, a sweeping response to Western sanctions imposed over its support for rebels in Ukraine.

The measures will hit consumers at home who rely on cheap imports, and on farmers in the West for whom Russia is a big market. Moscow is by far the biggest buyer of European fruit and vegetables and the second biggest importer of U.S. poultry.

RIA quoted the spokesman for Russia's food safety watchdog, the Veterinary and Phytosanitary Surveillance Service (VPSS), Alexei Alexeenko, as saying all European fruit and vegetables and all produce from the United States would be included in a ban drawn up on the orders of President Vladimir Putin to punish countries that imposed sanctions on Russia.

Earlier, Alexeenko told Reuters bans on EU and U.S. goods would be "quite substantial", and would specifically include U.S. poultry, although he declined to give a full list of banned goods. He could not be reached again after the RIA report.

The war of economic sanctions has escalated even as fighting has intensified on the ground in eastern Ukraine in the three weeks since a Malaysian airliner was shot down over territory held by pro-Russian rebels.

NATO said on Wednesday Russia had massed around 20,000 combat-ready troops on Ukraine's border and could use the pretext of a humanitarian mission to invade. It was the starkest warning yet from the Western alliance that Moscow could mount a ground assault on its neighbor.

As rebels have lost ground to Ukrainian government troops, Russia announced military exercises this week near the border.

"We're not going to guess what's on Russia's mind, but we can see what Russia is doing on the ground – and that is of great concern. Russia has amassed around 20,000 combat-ready troops on Ukraine’s eastern border," NATO spokeswoman Oana Lungescu said in an emailed statement.

Moscow could use "the pretext of a humanitarian or peace-keeping mission as an excuse to send troops into Eastern Ukraine", she said. A NATO military officer, speaking on condition of anonymity, said Russia's build-up at the border included tanks, infantry, artillery, air defense systems, logistics troops, special forces, and aircraft.

A Russian defence ministry spokesman dismissed the NATO accusations: "We've been hearing this for three months already."

Moscow annexed Ukraine's Crimea peninsula in the Black Sea in March, and Western countries say it has funded and armed pro-Russian rebels since they rose up in east Ukraine in April.

Over the past two months, government troops have fought back, gaining ground against the rebels, who are led almost exclusively by Russian citizens and have managed to acquire tanks, missiles and other heavy weaponry that Kiev and its Western allies say can only have come from across the frontier.

Kiev said 18 Ukrainian servicemen had been killed and 54 injured in 25 separate clashes over the past day in eastern Ukraine. Military spokesman Andriy Lysenko said troops had been shelled from inside Russian territory and frontier guards had come under a four-hour mortar and artillery attack.

Fighting has intensified since Malaysia Airlines flight MH17 crashed on July 17, killing all 298 people on board. Western countries say it was shot down by rebels with an advanced anti-aircraft missile supplied by Russia. Moscow denies blame, and the Russians who command the rebels deny they had such missiles.

The United States and the EU imposed sanctions on Russia that were mild at first but have been tightened sharply since the airliner was brought down, now targeting Russia's defense, oil and financial sectors.

On Wednesday, Putin ordered his government to come up with a list of agricultural products from countries that had imposed sanctions on Russia, which would be banned in retaliation. He told the government to avoid measures that would hurt Russian consumers, but the blanket bans reported by RIA were about as sweeping as could be.

Russia imported $43 billion worth of food last year. According to the European Commission, Russia bought 28 percent of EU fruit exports and 21.5 percent of its vegetables in 2011.

It was the second biggest buyer of U.S. poultry after Mexico last year, accounting for 8 percent of U.S. chicken meat exports, according to the U.S. Department of Agriculture. U.S. poultry has been ubiquitous in Russia since the early days after the Soviet Union, when cheap American chicken quarters sold at street markets were called "Bush's legs" after the president.

Siege

Kiev's military offensive has pushed the rebels out of many of their strongholds, leaving them largely besieged in the cities of Donetsk and Luhansk, which the rebels have proclaimed capitals of two "people's republics".