Japan's prime minister, Shinzo Abe, has told workers at the Fukushima Daiichi nuclear power plant that "the future of Japan" depends on their ongoing struggle to contain leaks of highly radioactive water at the site.

Abe's brief visit to the stricken plant on Thursday – his second since he became prime minister last December – comes weeks after he reassured the world that the situation at the facility was under control, amid reports that large quantities of contaminated water were seeping into the Pacific ocean.

Abe's reassurances are thought to have helped Tokyo's successful bid to host the 2020 Olympics, but were later challenged by a senior official at the plant's operator, Tokyo Electric Power [Tepco].

On Thursday, Abe and his entourage, dressed in protective suits, masks and helmets, heard Tepco officials explain how they planned to prevent additional leaks from tanks that have been hastily built to store water that becomes contaminated after it comes into contact with melted nuclear fuel in damaged reactor basements.

He also visited a water decontamination facility and a chemical dam being built along the coastline to contain leaks of groundwater into the Pacific ocean.

The visit was designed to calm fears at home and overseas about safety at the plant, amid rising doubts about Tepco's ability to conduct the cleanup operation alone.

This week it emerged that US experts had urged Japanese authorities to take immediate steps to prevent groundwater contamination two years ago, but their advice been ignored.

Tepco reportedly lobbied against the proposed construction of a barrier – a measure that will now be taken with government funding – because it feared the high cost would spook investors and push the firm closer to insolvency.

Charles Casto, a representative of the US Nuclear Regulatory Commission, said discussions about a barrier had begun within weeks of the meltdown.

"It was obvious to us that there was great deal of groundwater intrusion into the plant, and we shared that with the Japanese government," he told Reuters. "At the time, they didn't believe there was a significant amount of groundwater getting into the plant."

Abe told some of the thousands of workers at the plant that the government would continue to support the utility during a long potentially hazardous decommissioning operation that is expected to last four decades.

"The future of Japan is on your shoulders," he said during a visit to the plant's command centre. "The government will step forward and take concrete measures. I am counting on you to do your best."

Later, he said he had told Tepco officials, including the firm's president, Naomi Hirose, to decommission reactors 5 and 6, which were not in operation when the plant was wrecked by a powerful earthquake and tsunami on 11 March 2011.

Hirose said Tepco, which is hoping to restart some nuclear reactors to help repair its tattered finances, would make a decision on the two reactors by the end of the year. Reactors 1 to 4 have already been earmarked for decommissioning.

Abe stood by his recent claim, made before members of the International Olympic Committee in Buenos Aires earlier this month, that contaminated water had been prevented from flowing beyond the plant's harbour, and that the water crisis was under control.

"One of the main purposes of this visit was to see the situation for myself, having made those remarks on how the contaminated water is being been handled," he said, adding that the government believed the water issue would be resolved by the time the Games are held in 2020.

"As I stated in Buenos Aires, I am convinced that the contaminated water leaks have been confined to an area of 0.3 sq km within the cove next to the plant.

"In light of that, I will work hard to counter rumours questioning the safety of the Fukushima plant."

Earlier this week, however, Kazuhiko Yamashita, a senior Tepco official, said the water leaks were not under control.

"Predictable risks are under control, but what cannot be predicted is happening," Japanese media quoted Yamashita as telling opposition MPs.

"We believe that the current conditions show that [the radioactive water problem] is not under control."

Abe, accompanied by Tepco officials, was shown a water treatment facility that can remove radioactive materials from contaminated water. The equipment failed during an earlier a trial run and is now under repair. The plant's manager, Akira Ono, said it would be tested again later this year.

Abe later inspected a water tank that last month leaked 300 tonnes of water, sending radiation levels in the immediate vicinity soaring.

Ono said 90 Tepco workers were patrolling the plant's 1,000 water tanks four times a day, adding that gauges would soon be installed to monitor water levels in the tanks.

Tepco hopes to replace suspect tanks with more reliable welded versions by the end of the year, he added.