A leaked classified document alleges that Serbia, the only country in Europe that has refused to sanction Russia for its invasion of Ukraine, has agreed to supply arms to Kyiv or has sent them already.

According to Reuters, the document, a summary of European governments' responses to Ukraine's requests for military training and "lethal aid" or weapons, was among dozens of classified documents posted online in recent weeks in what could be the most serious leak of US secrets in years.

Entitled "Europe|Response to Ongoing Russia-Ukraine Conflict", the document in chart form lists the "assessed positions" of 38 European governments in response to Ukraine's requests for military assistance.

The chart showed that Serbia declined to provide training to Ukrainian forces, but had committed to sending lethal aid or had supplied it already. It also said Serbia had the political will and military ability to provide weapons to Ukraine in the future.

The document is marked Secret and NOFORN, prohibiting its distribution to foreign intelligence services and militaries. It is dated March 2, and embossed with the seal of the office of the Joint Chiefs of Staff.

Serbian President Aleksandar Vučić's office and the Ukrainian embassy did not immediately respond to a request for comment.

Vučić's government has professed neutrality in the Ukraine war, despite the country's deep historic, economic and cultural ties with Russia.

"If this document is accurate, it either shows Vučić’s duplicity vis a vis Russia or he's under enormous pressure from Washington to deliver weapons to Ukraine," said Janusz Bugajski, Eastern European expert with the Jamestown Foundation, a foreign policy institute.

The Pentagon chart divided the responses to Ukraine's requests for aid into four categories: countries that had committed to provide training and lethal aid, countries that had already provided training, lethal aid or both, and countries with the military ability and the political will "to provide future lethal aid".

Austria and Malta were the only two countries marked "No" in all four categories.

The disclosure of the chart comes just over a month after documents posted in a pro-Russian Telegram channel purportedly showed the shipment by a Serbian arms maker of 122mm Grad ground-to-ground rockets to Kyiv in November 2023. The documents included a shipment manifest and a Ukrainian government end-user certificate.

In March, Moscow said it had asked Belgrade to provide an official explanation for alleged deliveries of military aid.

Arms manufacturer Krusik Corp. from Valevo denied providing rockets or other weapons to Ukraine. Vučić called these accusations "blatant lies".

"We have not sent any weapons or ammunition to Russia or Ukraine," Vučić said during a visit to Qatar on March 5.