Finance Minister Taro Aso vowed Tuesday to look into why documents related to the controversial sale of state-owned land were doctored, as unconvinced opposition parties ratcheted up their criticism against the government.

A day after the Finance Ministry said it found 14 altered documents, Aso reiterated his intention to stay on. Cabinet ministers said the latest revelation will hurt public trust in the government but refrained from saying whether Aso should be held responsible.

"As we continue to investigate the case so something like this will never happen again, it's important to regain trust," Aso told reporters after a Cabinet meeting.

"My job as finance minister is to identify why it happened and prevent a similar incident in the future," said Aso, who also serves as deputy prime minister.

The document falsification once again shed light on a scandal that has haunted the administration and eroded public support since last year. At the heart of it is the sale at a discount of state property to school operator Moritomo Gakuen with ties to Prime Minister Shinzo Abe's wife.

Abe, who has denied his or his wife's involvement in the controversial land sale deal, said Monday he felt responsible as head of the government after the ministry admitted to having doctored documents.

In yet another revelation, the ministry found a note, originally attached to one of the 14 documents on the land deal, removed.

The ministry said late Tuesday the note on leasing the land to Moritomo Gakuen was apparently taken out by the Kinki local branch that was in charge of negotiating the deal with the school operator. The note's removal took place around June 2015 after the school operator demanded information disclosure.

The Finance Ministry also confirmed it was aware of the document falsification after the Ministry of Land, Infrastructure, Transport and Tourism alerted it on March 5 there were different versions of documents to those submitted to the parliament.

However, the Finance Ministry had refrained from acknowledging the document doctoring for a week until Monday because it needed time to grasp the whole picture of the problem, according to the ministry.

"It's the responsibility of a politician to oversee bureaucrats so they will not alter state documents and prevent a recurrence," Economy, Trade and Industry Minister Hiroshige Seko told reporters earlier in the day.

"The government is built on the trust of the Japanese people," Seko said.

Internal affairs minister Seiko Noda called the latest revelation regrettable and frustrating. "I'd like to ask everyone at the Finance Ministry to seriously reflect on this issue and get to the bottom of it," Noda said.

The ministry's probe found references to Abe and his wife Akie deleted. Aso said the 14 documents were changed to make them consistent with what a senior official of the ministry's Financial Bureau that had overseen the sale told parliament during grilling by opposition parties.

In protest, opposition parties were absent on Tuesday from a public hearing by the Budget Committee of the upper house.

The ruling bloc of the Liberal Democratic Party and the Komeito party is hoping to end the stalemate in the Diet and start deliberations on the budget and other bills. But lawmakers from the main opposition Constitutional Democratic Party of Japan have turned up the pressure on the administration, calling on Aso to take responsibility.

The altering of the documents "was purposely meant to deceive the Japanese people," said the party's leader Yukio Edano, adding that democracy is being threatened.

Edano called for the prime minister's wife and Nobuhisa Sagawa, the senior ministry official who later headed the National Tax Agency but resigned over the latest scandal, to testify before the Diet.

Edano said it was imperative for them "to tell the truth in parliament."

The ministry said again late Tuesday that the falsification was made unilaterally by the Financial Bureau.