LONDON — Arsonists probably started the wildfires that killed at least 84 people in Greece this week and nearly obliterated a seaside town, Prime Minister Alexis Tsipras said on Friday. The fires were the deadliest disaster in the country’s recent history.

After the three days of national mourning declared by Mr. Tsipras expired, political tensions rose along with the death toll, as questions persisted about who was responsible and how well the government responded to the blazes.

“I assume full political responsibility for the tragedy,” Mr. Tsipras said in an emergency cabinet meeting on Friday afternoon, and he urged the ministers of his government to do the same, “as heavy as that may be.”

But the prime minister failed to specify how the government mechanisms for handling such a disaster might have failed, and instead he placed the blame on poor urban planning and arbitrary housing in Mati, a coastal village east of Athens that was all but wiped out by fire on Monday and Tuesday.