Julian Borger in Washington

Under intense questioning about why the Israeli annexation of the Golan Heights was good but the Russian seizure of Crimea was bad, the US secretary of state, Mike Pompeo, told senators that there was an “international law doctrine” which would be explained to them later.

It turned out there was no doctrine. The state department’s clarification of Pompeo’s remarks contained no reference to one, and experts on international law said that none exists.

Donald Trump’s decision last month to recognise Israeli sovereignty over the Golan, captured from Syria in 1967, took the state department by surprise, and it has been struggling to catch up since.

Pompeo has provided several justifications for the move, from ancient Jewish roots on the plateau, to the justness of the Israeli cause in the “six-day war”, and the blunt force of “facts on the ground”.

On Tuesday, bombarded by senators’ questions on the distinction between the Golan and the Crimean cases, he suggested there was a body of international law underpinning Trump’s move, that would soon be revealed.

“There is international law doctrine on this very point. We don’t have time to go through it today. But [I’m] happy to have a team go over and walk you through that element of international law,” Pompeo told a Senate appropriations subcommittee.

Asked for clarification about the “doctrine” the state department issued a statement that made several arguments but no legal ones.

“Israel’s administration of the Golan Heights, and Russia’s occupation and purported annexation of Crimea should not be compared, as the circumstances couldn’t be more different,” the statement said.

“Israel gained control of the Golan through its legitimate response to Syrian aggression aimed at Israel’s destruction,” it continued. “Russia has occupied Crimea despite the fact that it has recognized Crimea as part of Ukraine in bilateral agreements, and despite its international obligations and commitments, including core OSCE [Organization for Security and Cooperation in Europe] principles.”

The statement concluded by saying: “The US policy continues to be that no country can change the borders of another by force.” It did not explain how that statement squared with the recognition of Israeli possession of the Golan. It turned out that the statement had first been issued on 26 March and the state department was still issuing it in response to all queries about the legal underpinnings of Trump’s recognition of the Golan annexation.

Experts on international law said the statement underlines the illegality of Russia’s seizure of Crimea, but attempts no legal justification for accepting Israel’s annexation of the Golan Heights.

Oona Hathaway, an international law professor at Yale University, said: “The international law is clear: there is no right to annex territory from another state by force, whether in an aggressive or defensive war.

“The distinction that the state department is drawing is absolutely false and entirely contradicted by longstanding international law,” Hathaway said. “The argument the administration is making to justify the annexation is outrageous and potentially destabilizing to the postwar international order.”

Israeli tourists stand near silhouetted cutouts of Israeli soldiers at Ben Tal, next to the Israeli- Syrian border in the Golan Heights last month. Photograph: Atef Safadi/EPA

The main bulwarks of international law on annexation are the UN Charter, and UN security council resolution 242, agreed in the aftermath of the 1967 Arab-Israeli war, which stressed “the inadmissibility of the acquisition of territory by war”.

It called for the “withdrawal of Israeli armed forces from territories occupied in the recent conflict” and respect for and acknowledgment of the “sovereignty, territorial integrity and political independence of every state in the area and their right to live in peace within secure and recognized boundaries free from threats or acts of force”.

Trump’s decision to recognise Israeli sovereignty over the Golan took the state department by surprise, and it has wavered in its justification of the decision.

As well as trying to make a distinction between land grabs as an outcome of defensive and offensive wars Pompeo has pointed to ancient Jewish links with the area, and sought to make a virtue of acceptance of “facts on the ground”.

A state department tweet on Tuesday cited Pompeo as saying: “The Trump administration sees the world as it is, not as we wish it would be. Basing policy on reality, we recognized Jerusalem as Israel’s capital [and] Israel’s sovereignty over the Golan Heights.”