Donald Trump's nominee for secretary of state allegedly "exacerbated" the already perilous situation of Christians in the Middle East, a top Democrat has claimed.

Rex Tillerson, the president-elect's choice for secretary of state, is the former chief of oil giant Exxon Mobil.

Rep. Jan Schakowsky (D-Ill.) accused the ex-tycoon of marginalising Iraqi Christian communities and acting against the US policy of the time.

In 2011 Exxon Mobil signed a deal with the unofficial Kurdistan Regional Government (KRG) prompting fury in both the American and Iraqi government.

But as well as undercutting governments, Tillerson's deal also threatened the Iraqi Christian community on the Ninevah Plain, according to Schakowsky.

"The number of Christians in Iraq has declined from 1.2 million residents in 2003 to less than 250,000 today," she wrote in a letter to the Forerign Relations Committee.

"The deals Mr. Tillerson signed exacerbated an already perilous situation for those beleaguered communities. He helped further marginalise the Assyrian/Chaldean/Syriac Christians and embolden the territorial expansion into their ancestral homeland ― all with neither consent or input from nor compensation to the indigenous communities of the Nineveh Plain."

Schakowsky's letter urged the Senate Foreign Relations committee to quiz Tillerson on the deal and whether he will promise "to uphold United States policy of protecting indigenous communities such as the Assyrian/Chaldean/Syriac Christians in the Ninevah Plain?"

She also asked whether Tillerson would, as secretary of state, "protect vulnerable communities such as those in the Ninevah Plain from private companies and governments who put profit over human safety and security?"

The controversial deal caused outrage among US officials because it was signed with the semi-autonmous Kurdistan region rather the main Iraqi government, undercutting US foreign policy.

Then US-ambassador to Iraq, James Jeffrey, "dropped a few F-bombs" when he learnt of the deal, according to a Reuters report.

The Kurdish authorities have encroached on the Christian's land on the disputed Ninevah Plain and subject the local believers to "pervasive discrimination and marginalization," according to the US Commission on International Religious Freedom.

Demand for Action, a charity fighting on behalf of Middle Eastern Christians, have urged US politicians to press Tillerson on his role in the deal and whether any compensation was offered to the local villages.

Chief executive Steve Oshana told Christian Today: "As CEO of ExxonMobil Rex Tillerson showed a pattern of disregarding American foreign policy and putting profit ahead of all else. While some may argue this was his duty as a CEO, he also has a duty as an American.

"As our nation's top diplomat, if confirmed Mr. Tillerson will be charged with the Holy task of defending the rights of my people, and we will never waiver in holding anyone accountable when their actions, whether in the public or private sector, harms our people in our ancestral homeland."

He added according to Huffington Post: "Here is a situation where, with the machinery of an American company, more Assyrian resources are being taken ... what this action does is it says to our community and the world that our indigenous rights are secondary to the oil rights of everyone else."

He said: "We're not trying to stop the confirmation. However, as he's rapidly going into his role, this is our opportunity to remind Mr. Tillerson that the actions that he took as a CEO had serious ramifications in marginalizing Christians, which is a group Donald Trump has vowed to help.

"He's somebody who has had an active role in marginalising our communities. That's something people have forgiven [of him] as a CEO, but as secretary of state, he has to live up to Trump's promise," Oshana said.

"If [Trump] really cares about helping our people, then this is a slam dunk."

Christian Today has contacted Exxon Mobil for comment.