North Korea's leader Kim Jung Un has unexpectedly extended his visit to Russia, where he was meeting President Vladimir Putin for a suspected arms deal.
The pair had discussed possibilities for military cooperation on Wednesday.
Mr Putin also "gratefully" accepted an invitation from Mr Kim to visit North Korea, a Kremlin spokesperson said.
Moscow is buying weapons for its war on Ukraine and any help would violate UN resolutions, the US has warned.
Mr Kim was warmly received by Mr Putin at the Vostochny space centre on Wednesday in Russia's far east - the North Korean leader spent two days travelling there in his private luxury bulletproof train.
Russian state media footage showed the two leaders grinning as they shook hands, before Mr Putin personally escorted Mr Kim around the space centre.
They also exchanged gifts, Kremlin spokesman Dmitry Peskov said. Mr Putin gave Mr Kim a glove from a spacesuit "that had been in space several times" and a Russian-made rifle. Mr Kim gave Mr Putin a North Korean-made gun, among other gifts.
Mr Putin returned to Moscow after the summit, but Mr Kim's visit would continue for several days, Mr Peskov added without elaborating further.
The North Korean leader was expected to oversee a display of Russian warships, as well as visit several factories and stop by the eastern city of Vladivostok on his way home.
Wednesday's meeting between the two sanctioned regimes took place at a time when their relations with the West are at an all-time low.
The two leaders discussed military matters and the war in Ukraine during the summit - which Mr Kim appeared to express support for.
"Russia has risen to a sacred fight to protect its sovereignty and security against the hegemonic forces" of the West, Mr Kim said, telling Mr Putin he would "always support" his decisions.
Mr Putin also said he would help Pyongyang develop satellites, drawing concern in the US that Russian help with satellite technology would improve the North Korean missile programme.
"That is quite troubling and would potentially be in violation of multiple UN Security Council resolutions" which Russia itself had voted for in the past, US State Department spokesman Matthew Miller said on Wednesday.
Mr Putin appeared to acknowledge this, saying there were "there are certain limitations" to military co-operation.
The US has also warned that it would "not hesitate to take action" if North Korea provides weapons to Russia, to which the Kremlin had said that the interests of Russia and North Korea were important to them "and not warnings from Washington".