The ship set sail from the Lebanese port of Tripoli
A Lebanese ship carrying aid for Gaza has been stopped by the Israeli navy and taken under escort to the Israeli port of Ashdod, Israeli officials say.
Earlier, officials in Lebanon said Israeli gunboats had fired on the ship before soldiers boarded it, although Israel denied this.
Lebanese PM Fouad Siniora has called on the international community to persuade Israel to allow the shipment through.
In the West Bank, Israeli troops have killed a Palestinian militant.
The Israeli military said Ala a-Din Abu Rop was a local commander of the armed wing of the Islamic Jihad group.
There have been sporadic incidents of violence since Hamas and Israel declared separate ceasefires on 18 January, following Israel's three-week attack on the Gaza Strip.
The Togo-flagged Tali aid ship was reported to have set off from the Lebanese port of Tripoli on Tuesday carrying 50 tonnes of medical supplies, food, clothing and toys for Gaza.
Ehud Barak said the aid ship "tried to slip into Gaza waters"
Also on board were eight activists and journalists, as well as the former Greek-Catholic archbishop of Jerusalem, Monsignor Hilarion Capucci, who had served time in an Israeli jail in the 1970s for smuggling arms for the Palestine Liberation Organisation (PLO).
An organiser of the shipment, Maen Bashur, said the ship had been confronted by an Israeli military boat 32km (19 miles) off the Gazan coast late on Wednesday.
"We were informed by the crew that the Israeli forces boarded the ship after firing shots at it," he told the AFP news agency. "We have lost contact with them."
He said the ship was asked to turn back as "two Israeli military helicopters flew over the area and fired flares".
The Israeli military denied troops had fired at the ship. "No gunshots were fired on board during the boarding and capture of the cargo boat," it said.
Israeli Defence Minister Ehud Barak said the Israeli navy had requested the ship head towards Egypt, and only boarded it after it headed back towards Gaza.
"At first the ship understood we were prohibiting it from heading to Gaza and steered towards El-Arish [in Egypt]," he said.
"From Egyptian territorial waters it tried to slip into Gaza waters. That is when the Israeli navy boarded it."
The army said the crew of the ship would be questioned by police, while all humanitarian goods found on board would be transferred to Gaza by land
Ala a-Din Abu Rop was shot dead by Israeli troops during a raid on his home near Jenin, in the West Bank, on Thursday.
An Israeli military spokesman said the 21-year-old was suspected of involvement in attacks on Israelis.
He was fully armed when troops stormed his home, and weapons and ammunition were found during the search, the military said.
Ala a-Din Abu Rop's father said his son had been sleeping, alongside his brothers, when soldiers broke into the house before dawn.