The UN warned on Friday that aid for the Gaza Strip could grind to a halt in days, as Israeli troops took their ground war with Palestinian fighters into the crowded city of Rafah, a key aid corridor for the famine-threatened strip.

Israeli tanks captured the main road dividing the eastern and western sections of Rafah, effectively encircling the eastern part of the city in an assault that has caused Washington to block some military aid to its ally.

Residents described almost constant explosions and gunfire east and northeast of the city on the southern edge of the Gaza Strip on Friday, with intense fighting between Israeli forces and militants from Hamas and Islamic Jihad.

Hamas said it ambushed Israeli tanks near a mosque in the east of the city, a sign the Israelis had penetrated several kilometres from the east to the outskirts of the built-up area. Israel has ordered civilians out of the eastern part of Rafah, forcing tens of thousands of people to seek shelter outside the city, previously the last refuge of more than a million who fled other parts of the enclave during the war.

Israel says it cannot win the war without assaulting Rafah to root out thousands of Hamas fighters it believes are sheltering there. Hamas says it will fight to defend it.

Supplies were already running short and aid operations could halt within days as fuel and food stocks get used up, UN aid agencies said.

“For five days, no fuel and virtually no humanitarian aid entered the Gaza Strip, and we are scraping the bottom of the barrel,” said the Unicef Senior Emergency Coordinator in the Gaza Strip, Hamish Young.

Aid agencies say the battle has already put hundreds of thousands of already displaced civilians in harm’s way.

“It is not safe, all of Rafah isn’t safe as tank shells landed everywhere since yesterday,” Abu Hassan, 50, a resident of Tel al-Sultan west of Rafah told Reuters via a chat app.

“I am trying to leave but I can’t afford 2,000 shekels ($540) to buy a tent for my family,” he said. “There is an increased movement of people out of Rafah even from the western areas, though they were not designated as red zones by the occupation.

“The army is targeting all of Rafah not only the east with tank shells and air strikes.”

Israeli tanks have already sealed off eastern Rafah from the south, capturing and shutting the only crossing between the enclave and Egypt. An advance on Friday to the Salahuddin road that bisects the Gaza Strip completed the encirclement of the “red zone” where they have ordered residents out.