Kiev, Ukraine - Russia and Ukraine on Friday traded accusations of cross-border shelling amid rising tensions that threaten to escalate the conflict even further just as Ukrainian forces are making significant advances and forcing separatists to retreat.

The situation at the border is growing increasingly volatile, with what U.S. officials say are about 15,000 troops massed on the Russian side and indications that the heavily militarized border is about to get more so.

The Pentagon warned of an "imminent" delivery from Russia of sophisticated and powerful rocket launchers to pro-Russia separatists fighting government forces in eastern Ukraine. If true, it means Russia will be even more deeply involved in the conflict following Ukrainian military gains since the downing of a Malaysia Airlines passenger plane a week ago - despite the squeeze of growing international sanctions against Russian officials and companies.

"In the eight days since the plane was shot down, rather than take the opportunity to follow the political and diplomatic track to a resolution, what some in the Kremlin are doing is pouring gasoline on a fire," Geoff Pyatt, the U.S. ambassador to Ukraine, told reporters in Kiev on Friday.

The Russian Foreign Ministry accused the United States of conducting a "smear campaign" against Russia with "unsubstantiated public insinuations."

It said the United States had supported "a coup d'etat in Kiev, and then in fact urged the regime toward the brutal massacre of the Russian-speaking population." Washington, it added, shares full responsibility for the bloodshed in eastern Ukraine.

Russian authorities said Ukraine had purposely tried to kill investigators checking reports of cross-border shooting by firing up to 40 mortar rounds into the Russian province of Rostov.

"It was only the poor preparation of the Ukrainian military and the timely evacuation of law enforcement officers under the cover of armored transport vehicles that did not allow the shooters to realize their intention," said Russia's Investigative Committee in a statement that suggested Moscow could be trying to establish a pretext for greater involvement with the separatists - or an invasion.

In Kiev, Defense and Security Council spokesman Andriy Lysenko said Russian forces massed along the border had increasingly been firing at Ukrainian troops. He displayed a July 23 photograph showing the rubble of stone houses destroyed by what he said were shells lobbed from Russian territory.

Lysenko said that in the past two days alone, Ukraine shot down three Russian drones.

One had been conducting surveillance on a Ukrainian base near the town of Amvrosiivka, which was subjected to rocket attacks, he said.

The Ukrainian Security Service released a video Friday, also dated July 23, that purported to show a volley of shells from the Russian side of the border aimed at Ukrainian troops stationed near Grigorivka.

What appears to be Russia's expanding support of the rebels, including units led by Russian citizens, makes it increasingly clear that Ukraine already is engaged in a war not just with its own insurgents but with its mammoth neighbor.

In the week since Malaysia Airlines Flight 7 was shot down by a missile fired from separatist territory, the Ukrainian military has made significant strides, retaking 10 towns held by separatists. They have squeezed the rebels in an ever-tightening vise and are close to gaining control of some major roads between Donetsk and Luhansk, the two epicenters of insurgency.

Lysenko said the Ukrainian military had, after several days of fierce fighting, on Thursday night retaken the city of Lysychansk, northwest of the separatist stronghold Luhansk.

By some accounts, the Ukrainian military, which was undermanned and undertrained when rebels rose up this spring but has since improved with experience in the field, could overwhelm the rebels within two or three weeks if Russia keeps its distance and does not reinforce the separatists with more weapons and equipment.

"I assure you our operations will continue, and very soon we will liberate Luhansk and Donetsk," said Zoryan Shkiryak, an adviser to the Internal Affairs Ministry, in announcing a probe into two Russian politicians suspected of encouraging and financing separatist activities.

Moscow has repeatedly denied playing any role in supporting the separatists battling government troops. Government forces have suffered 325 deaths and 1,232 wounded; the number of rebel casualties is unknown. The fighting has displaced tens of thousands of people and resulted in extensive damage to water and electrical supplies in the east.

Daniel Baer, the American ambassador to the Organization for Security and Cooperation in Europe, on Friday expressed disappointment that Russia has agreed to international observers at only two small checkpoints at the Ukrainian border. He said that covers just one kilometer of a border that spans 2,300 kilometers - or more than 1,400 miles. That means, he said, that there will be no one to monitor the flow of Russian arms, people and money to the rebels in eastern Ukraine.

The continued fighting is posing a growing dilemma for the investigation of the Malaysia Airlines crash. The site remains unsecured, and investigators who fanned out Friday found more wreckage, human remains and even a boarding pass in wooded areas and towns that had been unexplored before.

Michael Bociurkiw, a spokesman for the OSCE, said passports and credit cards mysteriously appeared near the wreckage where they had not been the previous day.

In an interview with CNN, he said patience appears to be wearing thin among insurgents, who told investigators they have another week to complete their work. They advised him to pass the word up the chain of command the vast amounts of wreckage must be taken away, he said.

Many human remains still must be found. Deputy Prime Minister Volodymyr Groysman said Friday that four refrigerated railroad cars filled with body bags that were taken by train to the eastern city of Kharkiv on Tuesday have been emptied. The remains were placed in 227 caskets he said. That leaves 71 passengers and crew members unaccounted for, and their remains are presumably strewn around a large area where wreckage and bodies plummeted to earth.

The escalation on the battlefield also is occurring during an awkward period for the Ukrainian government. Prime Minister Arseniy Yatsenyuk announced his resignation Thursday, and Groysman is fulfilling his duties until parliament meets to decide whether to accept Yatsenyuk's departure.

Parliament adjourned Thursday for a two-week break without voting on Yatsenyuk's resignation. But parliamentary leaders said they will recall the lawmakers for an extraordinary session July 31.

In a sign of growing European outrage over Russia's support for the rebels, the European Union on Friday banned visas and froze the assets of 15 more people and 18 more companies and organizations.

Washington Post staff writer Karoun Demirjian in Moscow, Post correspondent Alex Ryabchyn in Kiev and Post staff writer Karen DeYoung in Washington contributed to this report.