LUHANSK, Ukraine -- Tensions between Russia and Ukraine escalated sharply on Friday as Moscow sent more than 130 trucks rolling across the border in what it said was a mission to deliver humanitarian aid. Ukraine called it a "direct invasion," and the U.S. and NATO condemned it as well.

In another ominous turn in the crisis, NATO said it has mounting evidence that Russian troops are operating inside Ukraine and launching artillery attacks from Ukrainian soil -- significantly deeper involvement in the fighting than the West has previously alleged.

The trucks, part of a convoy of 260 vehicles, entered Ukraine without government permission after being held up at the border for a week amid fears that the mission was a Kremlin ploy to help the pro-Russian separatists in eastern Ukraine.

By late afternoon, trucks had reached the city of Luhansk, whose war-reduced population of a quarter-million people has suffered under intense fighting over the past several weeks between Ukrainian forces and the separatists.

Russia said the white-tarped vehicles were carrying food, water, generators and sleeping bags.

Some of the trucks were opened to reporters a few days ago, and at least some of those items could be seen. But Associated Press journalists following the convoy across rough country roads heard the trucks' contents rattling and sliding around Friday, suggesting many vehicles were only partially loaded.

The arrival of the trucks instantly raised the stakes in the crisis: An attack on the convoy could give Russia a pretext to intervene more deeply in the war. And a pause in the fighting to allow the convoy safe passage could hamper further battlefield advances by Ukrainian forces, which have reported substantial inroads against the rebels over the past week.

In sending in the convoy, Russia said it had lost patience with Ukraine's stalling tactics and claimed that soon "there will no longer be anyone left to help" in Luhansk, where weeks of heavy shelling have cut off power, water and phone service and made food scarce.

At the United Nations in New York, Russian Ambassador Vitaly Churkin hotly denied any Russian troops were inside Ukraine. Russia has also steadfastly denied supporting and arming the rebels.

Moscow's decision to move unilaterally, without Red Cross involvement, raised questions about its intentions.

Suspicions were running high that the humanitarian operation may instead be aimed at halting Kyiv's momentum on the battlefield.

Ukrainian Prime Minister Arseniy Yatsenyuk declared that the trucks were half-empty and were not going to deliver aid but would instead be used to create a provocation. He said Russia would attack the convoy itself, creating an international incident.

Ukrainian security services chief Valentyn Nalyvaichenko called the convoy a "direct invasion" and said the half-empty trucks would be used to transport weapons to rebels and spirit away the bodies of Russian fighters killed in eastern Ukraine. He said the men operating the trucks were Russian military personnel trained to drive combat vehicles, tanks and artillery.

Nalyvaichenko insisted, however, that Ukraine would not shell the convoy.

NATO's secretary-general condemned Russia for sending in a "so-called humanitarian convoy." Anders Fogh Rasmussen said Russia committed "a blatant breach" of its international commitments and "a further violation of Ukraine's sovereignty."

The Pentagon demanded Russia withdraw the convoy immediately, warning: "Failure to do so will result in additional costs and isolation."

NATO spokeswoman Oana Lungescu said that since mid-August, the military alliance has seen multiple reports of direct involvement of Russian forces in Ukraine, along with transfers of tanks and other heavy weapons to the separatists, and "an alarming build-up of Russian ground and air forces in the vicinity of Ukraine."

"Russian artillery support -- both cross-border and from within Ukraine -- is being employed against the Ukrainian armed forces," she said. Previously, the West accused Russia of cross-border shelling only.

The Red Cross, which had planned to escort the convoy to assuage fears that it was a cover for a Russian invasion, said it had not received enough security guarantees to do so, as shelling had continued overnight. Four troops were killed and 23 wounded in a 24-hour period in eastern Ukraine, the government reported Friday.

The government said it had authorized the entry of only 35 trucks. But the number of Russian vehicles seen passing through was clearly way beyond that. International monitors said that as of midday, 134 trucks, 12 support vehicles and one ambulance had crossed into Ukraine.

In announcing its decision to act, the Russian Foreign Ministry said: "There is increasingly a sense that the Ukrainian leaders are deliberately dragging out the delivery of the humanitarian load until there is a situation in which there will no longer be anyone left to help."

It added: "We are warning against any attempts to thwart this purely humanitarian mission."

Rebel forces took advantage of Ukraine's promise not to shell the convoy to drive on the same country road as the trucks. Some 20 green military supply vehicles -- flatbed trucks and fuel tankers -- were seen travelling in the opposite direction, along with smaller rebel vehicles.

The fighting in eastern Ukraine began in mid-April, a month after Russia annexed Ukraine's Crimean Peninsula. It has killed over 2,000 people and forced 340,000 to flee, according to the United Nations.

On Friday, Lithuanian Foreign Minister Linas Linkevicius said the country's honorary consul in Luhansk had been abducted and killed by "terrorists." There were no further details.

Laura Mills in Moscow, Nataliya Vasilyeva in Kyiv, Ukraine, Peter Leonard in Donetsk, Ukraine, and Alexander Roslyakov in Donetsk, Russia, contributed to this report.