A Libyan cargo ship carrying humanitarian supplies for Gaza has turned back before reaching the strip.
Palestinian officials said the ship was intercepted by the Israeli navy. Israeli officials denied there had been a confrontation.
Israel has recently tightened its economic blockade of Gaza after an increase in border clashes.
The Libyan ship was the first attempt by an Arab state to break the blockade of the Gaza Strip.
European and other pro-Palestinian activists have made three sea journeys from Cyprus and landed in Gaza.
"Navy ships approached the Libyan boat and ordered it on the radio to turn back, and so it did," Israeli foreign ministry spokesman Yigal Palmor said.
"Anyone wishing to transfer humanitarian aid into Gaza is welcome to do it in co-ordination with Israel and through the regular crossings. They can also contact Egypt."
Israel withdrew its forces and settlements from in 2005, but it and Egypt control Gaza's borders. Israel also controls the strip's airspace and coastline.
Egypt's only border crossing with Gaza, at Rafah, has been generally closed since June 2006 but has opened on a few occasions for humanitarian purposes.
Israel permitted limited aid and fuel deliveries into the Gaza Strip on Thursday. The strip has been closed to virtually all supplies for the past three weeks following an outbreak of clashes.
Shortages and power cuts in the territory have led the UN to describe conditions there as the "worst ever".
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