The Australian Border Force has “intervened” at the border to ask further questions of at least three Australians suspected of planning to travel to Israel to serve in the country’s military.

The government is also warning Australians who seek to serve with the armed forces of a foreign country “to carefully consider their legal obligations and ensure their conduct does not constitute a criminal offence”.

Official sources who did not wish to be identified noted that the Criminal Code does not prohibit Australians from “serving in or with the armed forces of a foreign country”, but this carve-out applies specifically to foreign-incursion offences.

It does not provide a blanket exemption to other commonwealth criminal offences, and the Australian federal police has the power to investigate war crimes and other serious crimes allegedly committed overseas.

A UN commission of inquiry report will be presented to the human rights council in Geneva on Wednesday and alleges that both Israel and Hamas have committed war crimes since the 7 October attacks and the resulting conflict in Gaza.

The report recommends that all countries “conduct investigations under domestic or universal jurisdiction on core international crimes committed during the current war”. The Israeli government has claimed the findings reflect a “political agenda against Israel”.

The Department of Home Affairs has revealed that it and the ABF “are aware of four Australian citizens who have travelled outside of Australia since 7 October 2023 and who were suspected to have departed Australia to serve or attempt to serve with the IDF”.

Responding to a freedom of information request, the department added: “The ABF intervened with three of the four Australian citizens suspected of departing for Israel since 7 October 2023.”

Guardian Australia understands that this language doesn’t mean the citizens were actively stopped or discouraged from travelling, but in this case refers to them being asked more detailed questions about their travel plans during customs checks.

A source familiar with the matter said the ABF does not specifically track individuals intending to serve in foreign militaries but “all movements across the border are screened using a range of intelligence-informed targeting techniques”.

A spokesperson for the home affairs department said the Australian government was “alert to the potential for Australians to travel to Israel and the Occupied Palestinian Territories and engage in hostilities”.

“The government encourages all Australians who seek to serve with the armed forces of a foreign country to carefully consider their legal obligations, and ensure their conduct does not constitute a criminal offence,” the spokesperson said.

This mirrors language used by both the attorney general, Mark Dreyfus, and the foreign affairs minister, Penny Wong, in responding to several recent petitions that called on the government to investigate Australians who served in the IDF amid mounting concerns about the humanitarian impact in Gaza.

Clear advice needed for Australians in IDF

Rawan Arraf, the principal lawyer and executive director of the Australian Centre for International Justice, said the government’s response to the petitions fell “drastically short” of what was needed.

Arraf said Australian authorities had an obligation “to investigate and prosecute allegations of international crimes” and should issue clear public warnings that people “may be subject to investigations for war crimes or other international crimes offences”.

Arraf said her legal centre had “identified about 20 individuals in Australia who have recently fought or are currently serving in the Israeli army”. She said she planned to make this information available to the AFP.

Guardian Australia is not suggesting that Australians have committed crimes during their service with the IDF – only that several groups are calling for their conduct to be subject to more scrutiny.

The Greens immigration spokesperson, David Shoebridge, accused the government of “a deliberate lack of transparency” over its response to the involvement of any Australians in the conflict, including a “lack of clarity around what ‘intervention’ means”.

The AFP acknowledges on its website that Australia “has jurisdiction to investigate core international crimes that occur offshore”, such as war crimes, crimes against humanity and genocide, but says “it is not usually practical for the AFP to do so”.

The president of the Australia Palestine Advocacy Network, Nasser Mashni, said the government must “present more robust advice for Australians within the IDF who may be at credible risk of being implicated in international crimes”.

He said the existing advice was “tepid” and “grossly inadequate, particularly considering that Israel is already under scrutiny for war crimes and genocide in multiple international court investigations”.

The international court of justice has yet to rule on South Africa’s allegations that Israel’s conduct in Gaza amounts to genocide, but an interim ruling ordered it to ensure “that its military does not commit any acts” that breach the genocide convention.

The Israeli government has rejected the genocide allegations as “false and outrageous” and has also accused the prosecutor of another body, the international criminal court, of “callously pouring gasoline on the fires of antisemitism that are raging across the world”.

The ICC prosecutor applied last month for arrest warrants for the Israeli prime minister, Benjamin Netanyahu, and the defence minister, Yoav Gallant, along with three Hamas leaders, alleging that they were responsible for war crimes and crimes against humanity.

Donald Rothwell, a professor of international law at the Australian National University, said the Australian government’s public advice “could be enhanced” to reflect the ICJ and ICC developments and Australia’s “capacity to prosecute Australians who are alleged to have committed war crimes”.

But Rothwell said one challenge to domestic investigations “would be the gathering of evidence to be able to bring war crimes charges and to be able to mount a successful prosecution”.

A spokesperson for the Attorney General’s Department said individuals were “encouraged to seek their own independent legal advice with regard to their own specific circumstances”.

The Israeli embassy was contacted for a response. The Executive Council of Australian Jewry declined to comment.