GAZA CITY, Gaza Strip — A Gaza cease-fire quickly unraveled on Friday as violence erupted in and around a southern town in the war-ravaged strip, with at least 40 Palestinians killed in Israeli shelling while the military said two soldiers were killed and an infantry officer was feared captured during fighting there.

Israel and Hamas accused each other of breaking the cease-fire, which had been announced by the U.S. and the U.N. and took effect at 8 a.m. Friday. The fighting broke out less than two hours later.

The breakdown of the cease-fire and the apparent capture of the Israeli soldier set the stage for a major escalation of the 25-day-old conflict, which has already devastated large swaths of the impoverished coastal area.

The White House condemned the apparent capture of the soldier near the southern Gaza town of Rafah, describing it as an “absolutely outrageous” action by Hamas. Deputy National Security Adviser Tony Blinken said the soldier must be released immediately.

Israeli Prime Minister Benjamin Netanyahu, meanwhile, told U.S. Secretary of State John Kerry in a telephone conversation that Palestinian militants had “unilaterally and grossly” violated the ceasefire and attacked Israeli soldiers after 9 a.m.

“Israel will take all necessary steps against those who call for our destruction and perpetrate terrorism against our citizens,” Netanyahu told Kerry, according to a statement from the prime minister’s office.