Malsin is awaiting the court ruling on his expulsion
A US citizen working as an editor for a the Palestinian news agency Maan is appealing against Israel's refusal to allow him entry at Tel Aviv airport.
Jared Malsin has been detained since he returned from a holiday in Prague on Tuesday evening, his colleagues said.
Israeli security officials said security concerns had arisen when he was questioned, and the Interior Ministry had refused him entry.
Maan said a court hearing due on Thursday has been postponed to Sunday.
It said the decision could "only be explained as a retaliatory measure for his reporting on Palestine".
Mr Malsin, who Maan says is Jewish, is the English language editor for the news agency.
Interior Ministry spokeswoman Sabin Hadad said Mr Malsin had been denied entry because he had refused to answer during questioning.
She said issues related to Mr Malsin's visa "could have been solved if he had co-operated," said Ms Hadad.
An official report on the questioning, which Maan said it had received from the court, accused Mr Malsin of failing to arrange the correct visa, but did not give details.
It said he was suspected of "exploiting the fact that he is Jewish to gain a visa".
This was apparently on the basis that, when seeking a visa extension previously, he had told Interior Ministry officials he was exploring the option of emigrating to Israel, but had written articles critical of the country.
By law Jews from around the world are eligible to emigrate to Israel.
The report also said Mr Malsin had refused to give the name of the friend he said he lived with in the West Bank.
On Wednesday, the Committee to Protect Journalists, based in New York, called the decision "unacceptable".
"Israel cannot hide behind the pretext of security to sideline journalists who have done nothing more than maintain an editorial line that the authorities dislike," said Mohamed Abdel Dayem, the New York based organisation's co-ordinator for the Middle East and North Africa.
Israeli government spokesman Mark Regev said that allegations that the decision was because of Mr Malsin's journalism were "simply absurd".
Mr Malsin and his partner, Faith Rowold, both in their twenties, were detained and interrogated for eight hours on trying to re-enter Israel after a holiday in Prague, Maan said.
It said that Ms Rowold was expelled early on Thursday morning, while Mr Malsin was granted a court hearing and allowed to stay until a verdict was reached.
A court hearing scheduled for Thursday has been postponed until Sunday.
George Hale, one of Mr Malsin's colleagues at Maan, said they had both been in the region for two years, Mr Malsin working for Maan and Ms Rowold volunteering for the Lutheran Church in Jerusalem.
Mr Hale said Mr Malsin was well known to Israeli military and government officials, who he spoke to regularly, and had even been offered access to Israeli military facilities in the West Bank.
"It's preposterous, this is a guy who a month ago was invited to tour a military base in a settlement," said Mr Hale.
He said Mr Malsin, a graduate of Yale University, had initially come to Israel on the Birthright programme, which funds visits to Israel for young Jewish Americans.
Mr Malsin had never overstayed a visa, except for his most recent one, which was a few days overdue and that he had been told by officials this did not matter, Mr Hale said.
Foreign nationals working or volunteering with Palestinian organisations in the West Bank often complain of difficulty obtaining visas.
Many are present on three-month tourist visas, which do not provide permission to work and may not be extended.
Maan news agency says it "scrupulously maintains its editorial independence and aims to promote access to information, freedom of expression, press freedom, and media pluralism in Palestine".