The prospect of Gazans crossing into Egypt from the border town of Rafah to escape a military assault would make the resolution of the Israeli-Palestinian conflict impossible and cause an "atrocious dilemma" for the people fleeing, the UN refugee chief said on Friday.

Filippo Grandi, United Nations High Commissioner for Refugees (UNHCR), said "we must fervently do everything" to avoid such an outflow of the Gazan population.

The prospect of Gazans crossing into Egypt from the border town of Rafah to escape a military assault would make the resolution of the Israeli-Palestinian conflict impossible and cause an "atrocious dilemma" for the people fleeing, the UN refugee chief said on Friday.

Filippo Grandi, United Nations High Commissioner for Refugees (UNHCR), said "we must fervently do everything" to avoid such an outflow of the Gazan population.

Even Israel's closest ally, the United States, has warned Prime Minister Benjamin Netanyahu that the country would face global isolation if it goes ahead.

Grandi said an attack on Rafah may make the movement of Gazans into Egypt "the only option for safety available."

"This dilemma is unacceptable and the responsibility to avoid this dilemma lies squarely in this particular case with Israel, the occupying power in Gaza," he said.

Grandi said UNHCR was stocking tents and supplies and working with countries in the region on coming up with their own contingency plans for the possible arrival of Gazans.

"We are looking at the region and that the possibility not only of the outflow, but also that the conflict may expand," Grandi said.

"But I repeat, we must not arrive at that atrocious dilemma, which would be really almost the end of the road for what is really important here: ultimate peace."