Mattis - who is set to leave his position on January 1, 2019 - also canceled a recently scheduled trip to Israel.

United States outgoing Secretary of Defence James Mattis blocked a massive US$500 million weapons deal between Israel and Croatia, according to an Axio report.

Due to existing aid relations between the United States and Israel, the latter is required to notify and seek permission from Washington prior to selling any fighter jets to a third party.

Two weeks ago, Israeli Prime Minister Benjamin Netanyahu contacted Mattis regarding the deal - prior to the defence secretary tendering his resignation, I24 News has reported, citing Israeli officials.

Mattis - who is set to leave his position on January 1, 2019 - also canceled a recently scheduled trip to Israel.

“For reasons we don’t fully understand, the [United States] hardened their conditions and, apparently, we misread their position on the deal. Practically, the F-16 deal with Croatia is dead and we don’t think it is possible to get an agreement that will reconcile the U.S. conditions and the Croatian demands in the tender,” the official stated, adding that the rejection has embarrassed Israel, who is now required “to apologize to Croatia for the deal falling apart.”

The longstanding Israeli ally surprisingly pulled the plug on the deal which is alleged to have involved 12 F-16 fighter jets, that Israel was making moves to modify to better specifications before delivering the weapons to Croatia.

Mattis, according to the report, promptly rejected the request from Netanyahu and accused Tel Aviv of attempting to sell the jets with advanced electronic systems that are superior to U.S.-made jets. Washington has instructed Israel to only sell the planes in their original condition.

1 \ Outgoing Defense Secretary James Mattis rejected a request from Israeli Prime Minister Benjamin Netanyahu to soften the United States' conditions for a $500 million arms deal between Israel and Croatia for the sale of 12 F-16 fighter jets, an Israeli official told me — Barak Ravid (@BarakRavid) December 27, 2018

Last October, the United States - who has been an unwavering supporter of Israel - approved a 10-year, US$38-billion aid package for Tel Aviv.

The aid package includes an annual US$3.3-billion allotment, placed into effect during the administration of former U.S. President Barack Obama, in 2016. Washington also approved a US$705-million budget for Israel’s missile programs, in 2018.

According to the terms of the 2016 deal, Israel will be supplied with free military material until 2028.

Additionally, in early October, the United States Congress further proposed a motion to legally enforce the aid package. The motion seeks to set up a special mechanism to provide the aid from the country’s annual budget without any disruption to the funding.