Ukraine’s air defences risk running out of missiles and ammunition within weeks, an apparent leak of Pentagon documents dating from February suggests, as US defence officials raced to trace how the files became public.

One of the documents, dated 23 February and marked “Secret”, outlines in detail how Ukraine’s Soviet-era S-300 air defence systems would be depleted by 2 May at the current usage rate. It is unclear if the usage rate has since changed.

The authenticity of the Pentagon documents has not been verified. In a statement on Sunday, the Pentagon said it was reviewing the validity of the photographed documents that “appear to contain sensitive and highly classified material”.

Ukraine’s Buk air defence systems, which it relies on along with the S-300 to protest vital sites from Russian air power, could run into trouble by mid-April, the New York Times reported, and air defences protecting troops on the front line could be “completely reduced” by 23 May.

Russian fighter jets and bombers gaining more opportunity to attack Ukrainian forces could prove a major challenge for Kyiv, the Times reported, citing senior military officials.

Colonel Yuri Ihnat, a spokesman for the Ukrainian Air Force, did not comment specifically on the information contained in the documents but told the Wall Street Journal that Ukraine faced serious challenges in finding the Soviet-designed ammunition for its crucial S-300 and the Buk batteries.

“If we lose the battle for the skies, the consequences for Ukraine will be very serious,” he told the paper. “This is not the time to procrastinate,” he said, urging western allies to speed up their assistance.

The apparent leaking of the large cache of classified Pentagon documents may have been from a US source, some US officials believe.

“The focus now is on this being a US leak, as many of the documents were only in US hands,” Michael Mulroy, a former senior Pentagon official, told Reuters in an interview.

Officials say the breadth of topics addressed in the documents, which touch on the war in Ukraine, China, the Middle East and Africa, suggest they may have been leaked by an American rather than an ally.

US officials said the investigation was in its early stages and those running it had not ruled out the possibility that pro-Russian elements were behind the leak, which is seen as one of the most serious security breaches since more than 700,000 documents, videos and diplomatic cables appeared on the WikiLeaks website in 2013.

The Russian embassy in Washington and the Kremlin did not respond to requests for comment.

Following disclosure of the leak, Reuters has reviewed more than 50 documents labelled “Secret” and “Top Secret” that first appeared last month on social media websites, beginning with Discord and 4Chan. While some of the documents were posted weeks ago, their existence was first reported on Friday by the New York Times.

Reuters has not independently verified the authenticity of the documents. Some giving battlefield casualty estimates from Ukraine appeared to have been altered to minimise Russian losses. It is not clear why at least one is marked unclassified but includes top secret information. Some documents are marked “NOFORN”, meaning they cannot be released to foreign nationals.

Two US officials told Reuters on Sunday that they had not ruled out that the documents may have been doctored to mislead investigators as to their origin or to disseminate false information that may harm US security interests.

The White House referred questions to the Pentagon.

The Pentagon has referred the issue to the Department of Justice, which has opened a criminal investigation.

Such closely-guarded information could be of great use to Russian forces, and Ukraine said its president and top security officials met on Friday to discuss ways to prevent leaks.

The apparent leak has had ramifications beyond Russia’s war on Ukraine.

Another document, marked “Top Secret” and from a CIA Intel update from 1 March, says the Mossad intelligence agency was encouraging protests against Israeli prime minister Benjamin Netanyahu’s plans to tighten controls on the supreme court.

On Sunday, Netanyahu’s office said: “The Mossad and its serving senior personnel have not engaged in the issue of the demonstrations at all and are dedicated to the value of service to the state that has guided the Mossad since its founding.”

While the Mossad’s purpose is not defined by law, the spy agency is not meant to wade into domestic political matters.

On Monday, South Korea’s main opposition party urged the government to verify the Pentagon documents, which include claims that the US attempted to eavesdrop on senior officials in Seoul over arms sales.

The floor leader of the Democratic party, Park Hong-keun, made the demand after reports that CIA operatives had been monitoring an internal discussion about concerns that artillery shells that South Korea planned to sell to the US could ultimately end up in Ukraine, according to the Yonhap news agency.

South Korea has backed international sanctions against Russia and provided non-lethal aid to Kyiv, but has stopped short of sending weapons.

Despite being a major arms exporter, Seoul has a long-standing policy of not selling weapons to countries at war – a stance that has led to charges that the South wants to avoid antagonising Russia, whose support it needs to pressure North Korea into abandoning its nuclear weapons.

Referring to the New York Times article, Park said: “If the report is true, it would be an action that can never be acceptable between allies of 70 years, and an infringement of sovereignty and diplomatic foul play that breaks bilateral trust head-on.”

About 28,500 US troops are based in South Korea as a deterrent against nuclear-armed North Korea, while forces from the US and the South regularly conduct joint military exercises.

Park asked the South Korean national assembly to convene meetings of its foreign affairs, intelligence and defence committees to look into the unverified claims, adding that Washington should issue a “polite apology” if they were found to be true, Yonhap said.

The Pentagon has not addressed the contents of any specific documents, including the apparent surveillance of allies.

Two US officials, speaking on condition of anonymity, said the military and intelligence agencies were looking at their processes for how widely some of the intelligence is shared internally.

One of the officials said investigators were looking at four or five theories, from a disgruntled employee to an insider threat who actively wanted to undermine US national security interests.

With Justin McCurry and Reuters