The US Internal Revenue Service (IRS) website has suffered a number of technical difficulties on what is the last day for Americans to file their tax returns.

A number of pages key to amending tax records or making payments have been down at various points in the day, although a cause has not yet been identified.

Tax Day, as it it known normally falls on 15 April, but that was a Sunday, while Monday was a holiday in Washington DC.

“Currently, certain IRS systems are experiencing technical difficulties,” an IRS spokesperson said. “Taxpayers should continue filing their tax returns as they normally would.”

IRS Acting Commissioner David Kautter testified during a House Oversight Hearing Tuesday that a number of systems are down at the moment and that the agency is working to resolve the issue.

The IRS typically recommends that taxpayers use electronic filing to avoid common mistakes. Online filing is quicker than posting a return — when the site works. Those who file electronically typically also get any refund faster.

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Some in Congress are calling on the IRS to help those affected by the glitches by not penalising them.

“Tax Day is already a stressful time for millions of Americans, even when everything goes right,” Richard Neal, the top Democrat on the House Ways and Means Committee, said in a statement. “Given this news, I hope that the IRS will make accommodations so that every taxpayer attempting to file today has a fair shot to do so without penalty.”

US Treasury Secretary Steve Mnuchin said extensions will be granted to those impacted when the site is up again.

“We'll make sure taxpayers have extensions once the system comes up to make sure they can use it and it in no way impacts people paying their taxes,” he told reporters in New Hampshire. “It was just a technical issue we're working through. A high volume technical issue that impacted the system.”

The outage will not affect President Donald Trump, who has asked for an extension for filing his 2017 income tax return. He is set to file a full return by the October extended deadline.

“The president filed an extension for his 2017 tax return, as do many Americans with complex returns. He will file his tax return by the extension deadline of October 15, 2018,” White House Press Secretary Sarah Sanders said in a statement.