The United States said on Wednesday that it had seen evidence that North Korea has sent 3000 troops to Russia for possible deployment in Ukraine, a move that the West is casting as a significant escalation of the Ukraine war.
Putin has hosted a three-day summit of the BRICS bloc of developing economies in the Russian city of Kazan. Some 36 countries were represented.
Asked by a reporter about satellite imagery showing North Korean troop movements, Putin said: "images are a serious thing. If there are images, then they reflect something".
But he said NATO officers and instructors were directly involved in the Ukraine war and it was the West that had escalated the Ukraine crisis.
"We know who is present there, from which European NATO countries, and how they carry out this work," Putin said.
The Kremlin chief specifically mentioned article four of the Russian partnership deal with North Korea, which deals with mutual defence.
"There is article four. We have never doubted in the least that the North Korean leadership takes our agreements seriously. But what we do within the framework of this article is our business," Putin said.
A North Korean representative to the United Nations in New York said the US and South Korean assertions about sending troops to Russia were "groundless rumours".
The United States believes at least 3000 North Korean troops are undergoing training at three military bases in eastern Russia, while Ukraine said North Korean units were already in the Kursk region.
This is the border region where Ukrainian forces mounted a surprise incursion earlier this year.
Putin said Russia's army was moving forward along all sections of the front in Ukraine, and had trapped a large number of Ukrainian troops in the Kursk region.
Asked by one of Russia's top Kremlin correspondents what he would be ready to consider to end the war, Putin said:
"We are ready to consider any options for peace agreements based on the realities that are taking shape on the ground. And I'm not ready for anything else."
Russia, which is advancing, controls about one fifth of Ukraine, including Crimea which it annexed in 2014, about 80 per cent of the Donbas - a coal-and-steel zone comprising the Donetsk and Luhansk regions - and over 70 per cent of the Zaporizhzhia and Kherson regions.
Ukraine has said it will not rest until all Russian troops are expelled from its territory.
Asked about a Wall Street Journal report that cited Republican presidential candidate Donald Trump as saying he had threatened Putin against going after Ukraine on an unspecified date in the past, Putin said he did not recall such a threat.
"You can threaten anyone. (But) it is pointless to threaten Russia because it simply invigorates us," Putin said.
Russian President Vladimir Putin is remaining coy about North Korean troops joining the Ukraine war. (AP PHOTO)
"But I do not recall such a conversation with Mr Trump."
The Journal quoted Trump as saying that he had warned Putin that if he went after Ukraine: "'I am going to hit you so hard, you're not even going to believe it. I'm going to hit you right in the middle of fricking Moscow.'"
Putin said it was wise not to take such statements seriously given the heat of the presidential election campaign but said that he felt Trump was sincere about his desire to end the war in Ukraine.
"It seems to me that he says this sincerely, and statements of this kind, from whomever they come, we certainly welcome," Putin said
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