In a statement, Mr Abe said: “I want to offer my condolences to the people who lost their precious lives and express my sympathy to those who have suffered because of this eruption. We will continue to put our full effort on the rescue operation. We will be on guard for further eruptions and protect the lives of citizens from this”.
Japan does not confirm deaths until a formal examination has been carried out, and NHK national news said the 26 people had suffered heart attacks.
The eruption of the 3,067-metre (10,062-feet) peak, 200 km (125 miles) west of Tokyo, took place as the popular hiking site was packed with climbers, including children.
Ten people have so far been confirmed dead in Japan’s first fatal volcanic eruption since 1991, and 63 have been injured, some with broken bones. Eight are missing, but officials said some of them could possibly be among those who perished.
More than 500 rescuers had been combing the summit, ploughing through knee-deep ash and passing mountain lodges with holes punched in their roofs by rocks shot out of the volcano.
Helicopters lifted laden stretchers one by one from the summit, before rescue efforts were abandoned. As on Sunday, the smell of sulphur strengthened at the peak, fanning fears of toxic fumes and forcing rescuers off the mountain.
“This was a steam-driven (eruption) and it has been said it was extremely difficult to predict,” Chief Cabinet Secretary Yoshihide Suga told a news conference.
Asked whether the eruption would require careful assessment of the restart at Sendai, Suga said: “I don’t think so.”