Ukraine is likely to get the access it has demanded to the site in southeastern Poland where a missile killed two people, the Polish president's top foreign policy advisor said on Thursday.
Warsaw says evidence from the scene points to the explosion being caused by a Ukrainian air defence missile that went astray, something Kyiv denies saying it has evidence of a "Russian trace" in the blast.
Polish President Andrzej Duda said on Wednesday that access to the site of the explosion would require the agreement of both countries leading the investigation, Poland and the United States.
"A Polish-American investigating team is on the site," Duda's adviser Jakub Kumoch told private broadcaster TVN 24.
"The Ukrainians asked for access to the site of the investigation. If both parties agree, and as far as I know there will be no objection from the American side, such access could be obtained soon."
The Head of Poland's National Security Bureau, Jacek Siewiera, also told private broadcaster RMF FM that as far as he was aware Duda was not opposed to having Ukrainian observers at the scene.
The explosion has stirred fear and disbelief among residents of Przewodow, a village of around 440 people located 6 kilometres (4 miles) from the Ukrainian border, raising concerns among locals that the most deadly conflict in Europe since World War Two could spill across the frontier at any time.
Kumoch said that Poland had video evidence regarding the blast.
"There are indeed film materials, I would not like to talk about the content of the film materials that I saw in classified mode, but of course the prime minister gave such information, such materials exist," Kumoch said.
"These are our normal photos from the border, where you can see certain things. You see shots over Ukraine, fighting over Ukraine, and at some point, in a very short time, you see a certain sequence of events," he added.
He said that from the missile debris, the depth of the crater and the amount of fuel used it was possible to calculate where it was fired from.
Kumoch did not provide any other details. He said that he wanted the Ukrainians to get acquainted with these materials first, and in contact with those conducting the investigation.
Poland has said it believes the missile was an S-300, an old Soviet-era rocket used by both Russia and Ukraine.
While Warsaw and Kyiv differ on the location the missile was fired from, they and other Western allies are united in the view that ultimately Russia is at fault for the incident as it resulted from its bombardment of Ukraine.
"The Russian side must be aware of the threat it poses by bombing...at a distance of literally several dozen kilometres from the Polish border, that any of the missiles, whether from the Russian side or those Ukrainian anti-missile systems, can land on the territory of a foreign state, in this case, Poland," said Adrian Kubicki, Poland's Consul General in New York.
"So nothing here changes the assessment that the Russian Federation is responsible for what happened."
A Reuters video journalist on the outskirts of the Przewodow said there appeared to be few locals on the streets on Thursday, other than children being taken to school by bus. Access to Przewodow has been blocked by police.
Military and police vehicles entered the village periodically, while soldiers patrolled the streets and surrounding fields.
Polish media reported that Duda was due to visit later in the day.
Przewodow is less than 10 kilometres away from one of the two power lines linking Poland and Ukraine. The cable was shipping Ukrainian electricity to Poland until Oct 11, when the flows stopped amid heavy Russian shelling that destroyed parts of energy infrastructure.