Ukrainian President Petro Poroshenko, speaking on Thursday after the army lost a key town to separatists, said the situation in the conflict zone in the east was "extraordinarily difficult ... but controllable", Interfax news agency reported.
Poroshenko, who was speaking at a meeting of top security chiefs after the town of Novoazovsk fell to Russian-backed separatists, said Russian troops had come "to the rescue" of the rebels after Kiev's earlier military successes.
The loss of Novoazovsk, Ukraine's most southeasterly point, after a two-day assault by Russian-backed separatists in an armoured column, is a blow to government forces since it leaves vulnerable the big port city of Mariupol, further west along the coast.
Separately, a military spokesman said Ukrainian forces had regrouped to defend Mariupol and had enough troops to stop it being taken.
"The Ukrainian military have reinforced and are working out plans for a counter-offensive," the spokesman, Andriy Lysenko, told journalists.
A presidential spokesman in a Twitter post said that Poroshenko's central message to his security chiefs was: "We are capable of defending ourselves. The main thing is not to panic."
Earlier on Thursday, a NATO military officer said the alliance believes well over 1,000 Russian troops are operating inside Ukraine.
"We assess well over 1,000 Russian troops are now operating inside Ukraine," the officer said at NATO's military headquarters in southern Belgium. "They are supporting separatists (and) fighting with them."
In turn, Ukraine's envoy to European security body the OSCE said regular Russian forces had seized the southeastern city of Novoazovsk, but his Russian counterpart said no Russian forces were crossing the border.
Ihor Prokopchuk, Ukraine's representative to the Organization for Security and Cooperation in Europe, told reporters that "what we registered is a direct invasion of the Russian military into the eastern regions of Ukraine". He said Ukraine viewed it as an "act of aggression".
Russian Ambassador Andrey Kelin separately told reporters during an extraordinary meeting of the OSCE: "No Russian forces are crossing in any point the border of Ukraine."
President Petro Poroshenko, in a statement explaining his decision to cancel a visit to Turkey, said: "Russian troops have actually been brought into Ukraine."
Russia denies intervening in Ukraine by arming the rebels or sending soldiers across the border. The defence ministry declined to comment on reports of Russian tanks in Novoazovsk.
"The Russian authorities clearly said many times there are no regular Russian troops there. Russia is not taking part in this armed conflict," said a Russian diplomatic source.
The latest escalation in the five-month crisis came only two days after the presidents of the two countries held their first talks in more than two months and agreed to work towards launching a peace process.
RUSSIA RIGHTS ACTIVISTS
More than 100 Russian soldiers were killed in eastern Ukraine in a battle this month while helping pro-Russian separatists fight Ukrainian troops, two members of the Russian presidential human rights council said on Thursday, citing accounts of eye-witness and relatives of the dead.
Ella Polyakova and Sergei Krivenko, both members of the council, said around 300 people were also injured in the violence on Aug. 13 near the town of Snizhnye in Donetsk Province, when a column of trucks full of ammunition they were driving was hit by a sustained volley of Grad missiles.
"A column of Russian soldiers was attacked by Grad rockets and more 100 people died. It all happened in the city of Snizhnye in Donetsk Province," said Krivenko speaking by telephone. Polyakova said she had also been given the same figure, of more than 100, for the number of Russian soldiers killed in the attack.
NATO, UKRAINE AMBASSADORS TO MEET
Ambassadors from the 28 NATO countries and Ukraine will hold an emergency meeting on Friday to discuss the situation in Ukraine, a NATO official said on Thursday.
The meeting is being held at Ukraine's request and comes after Ukraine accused Russia of bringing troops into the southeast of the country in support of pro-Moscow separatist rebels.
Ukraine is not a NATO member but meets regularly with NATO in a body called the NATO-Ukraine Commission.
REACTIONS
The United States is considering a number of options in response to Russian involvement in eastern Ukraine and said sanctions were the "most effective tool," U.S. State Department spokeswoman Jen Psaki said on Thursday.
"In terms of what we are considering, we have a range of tools at our disposal," Psaki said at a media briefing.
She said increased sanctions on Russia are "the most effective tool, the best tool."
"A military solution is not what we think is the appropriate approach so we're taking every tool that we can to see if we can reach a solution here through diplomatic means," Psaki said.
Psaki also said the United States has seen a "pattern of escalating aggression" by Russia in Ukraine.
As well, U.N. Secretary-General Ban Ki-moon is alarmed by reports that fighting in Ukraine has spread southward, which if confirmed would mark a "dangerous escalation" of the crisis, Ban's spokesman said on Thursday.
"The international community cannot allow the situation to escalate further nor can a continuation be allowed of the violence and destruction that the conflict has wrought in eastern Ukraine," U.N. spokesman Stephane Dujarric told reporters.
The U.N. Security Council will meet at 2 p.m. EDT (1800 GMT) to discuss the Ukraine crisis at the request of Lithuania.
For his part, British Prime Minister David Cameron said Russia must stop its tanks from entering Ukraine, warning Moscow it would face further consequences if a political solution to the crisis was not found.
"It is simply not enough to engage in talks in Minsk, while Russian tanks continue to roll over the border into Ukraine. Such activity must cease immediately," Cameron said in a statement.
"We urge Russia to pursue a different path and to find a political solution to this crisis. If Russia does not, then she should be in no doubt that there will be further consequences."
In turn, Poland's Foreign Minister Radoslaw Sikorski said Russia's actions in eastern Ukraine were an "aggression" that has created the most serious security crisis in Europe in decades.
"This for sure is an aggression," Sikorski told state radio. "Poland's position will be worked out in the next couple of hours. We do not need strong words here, but actions, best if actions of the whole international community."
Sikorski said the actions needed to be firm, but prudent, because the consequences could potentially be "unfathomable".
REUTERS