* Investigators say officers were clearly identifiable
* Russian regional official says Ukraine launched 30-40 mortar bombs
* U.S. ambassador to NATO says more than 15,000 Russian troops on border
* Russia rejects U.S. accusations over cross-border shooting (Adds U.S. ambassador to NATO on Russia troop numbers)
By Thomas Grove
MOSCOW, July 25 (Reuters) - Russian authorities accused Ukraine of trying to kill law enforcement officials who were checking reports of cross-border shooting on Friday by firing a volley of mortar rounds over the frontier into Russia.
A Russian security official said up to 40 mortar bombs fired by Ukrainian forces had fallen in the Russian province of Rostov near the border where Ukrainian government forces are fighting pro-Russian separatists.
There were no reports of injuries.
"Those who shot from Ukraine carried out the shooting purposefully with an intent to kill Russian law enforcement officials," said Russia's Investigative Committee, which answers only to President Vladimir Putin.
"It was only the poor preparation of the Ukrainian military and the timely evacuation of law enforcement officers under the cover of armoured transport vehicles that did not allow the shooters to realise their intention," it said in a statement.
Accusations by Ukraine's pro-European government, its Western backers and Russia over cross-border fire have ratcheted up tensions in the worst crisis between Moscow and the West since the end of the Cold War.
The U.S. ambassador to NATO, Douglas Lute, said Russia was again increasing troop numbers on its long border with Ukraine, where Washington and Kiev say Moscow is letting fighters and arms cross into rebel-held eastern Ukrainian territory.
Lute said the estimate was "now up again over about 15,000 Russian troops amassed along the border with Ukraine".
U.S. statements on Ukraine, which Moscow traditionally views as within its sphere of influence, have angered Russia. Its foreign ministry reacted angrily on Friday to U.S. accusations that Russian forces had fired artillery across the border targeting Ukrainian military positions.
"We deny the unfounded public insinuations that State Department spokeswoman (Marie) Harf repeats day after day," the ministry said in a statement.
"In journalistic briefings in previous days she has produced anti-Russian clichés that Washington stubbornly tries to impose on international opinion," the statement said.
The Investigative Committee said its officers were identifiable as law enforcement officials and were in Rostov province's Kuybyshevsky region to look into previous accounts of cross-border shelling.
The committee has thrust itself to the forefront of the crisis, accusing Ukrainian Interior Minister Arsen Avakov and Ihor Kolomoisky, a provincial governor, of "criminal acts" in Kiev's military push against the pro-Russian rebels.
Vasily Malayev, a regional representative of Russia's Federal Security Service branch devoted to border security, was quoted by Russian news agency Interfax as saying that 30 mortar rounds had landed in a village in the region.
He earlier told state Ria Novosti news agency around 40 shells had come across the border.
Ukraine says missiles shot from Russia may have downed two of its fighter jets this week. Moscow denies the accusation.
Russia's Chief of General Staff was quoted as saying by Interfax that it had proof that Ukraine had used phosphorus bombs that can kill through burns or smoke inhalation.
Earlier allegations of the use of the weapon were never proved and Kiev quickly denied this accusation.
"It's complete nonsense. We don't use phosphorus bombs. We simply don't have them. We use flares, but they have no relation to phosphorus bombs," said Ukrainian military spokesman Vladislav Seleznyov. (Additional reporting by Phil Stewart in ASPEN, Colorodo,; Editing by Gareth Jones)