Washington  - US president-elect Barack Obama's pick for Treasury secretary, Timothy Geithner, will have to wait until next week for a confirmation hearing in the Senate amid questions about his failure to pay taxes earlier this decade.

The Senate Finance Committee on Wednesday postponed its confirmation hearing until January 21, one day after Obama's inauguration, after Republicans on the committee objected to an earlier date.

The delay comes after problems arose Tuesday with Geithner's taxes and the immigration status of his former housekeeper. The committee had originally planned to interview Geithner on Friday.

Geithner failed to pay Social Security and Medicare taxes for himself while working for the International Monetary Fund. A complicated tax status required him to pay the taxes himself that are normally withheld from workers' paychecks by their employers.

He later paid four-years worth of back-taxes once the matter was brought to his attention. But some politicians are asking whether such a mistake is acceptable for the man who will take charge of the Internal Revenue Service, the agency which oversees tax collections for the country.

"Thats one of the very basic issues that have been raised by a lot of staff members and members of the Senate," Senator Charles Grassley, the top Republican on the Finance Committee, told Bloomberg News.

Many Republican senators also said they did not wish to delay Geithner's appointment to a position regarded as crucial for the country as it battles its way out of a recession.

Geithner, who now heads the Federal Reserve Bank of New York, also briefly employed a housekeeper whose immigration papers had expired while she worked for him, the Wall Street Journal reported Tuesday, citing people familiar with the matter.

She later got a green card and worked legally, but the employment of illegal immigrants has proved a major stumbling block for past government appointees.

Democratic senators, who hold a majority in the chamber, continued to express support for Geithner. Obama's transition team does not expect his nomination to be derailed.

Obama spokesman Robert Gibbs called the problems "honest mistakes" by Geithner that had since been addressed. (dpa)