By Ginger Gibson

WASHINGTON (Reuters) - U.S. Republican presidential nominee Donald Trump insisted on Monday he was not changing his stance on immigration, despite a comment from a top aide who said his plan to deport 11 million illegal immigrants was still under review.

"I'm not flip-flopping. We want to come up with a really fair but firm answer," Trump told Fox News. He did not elaborate on his policy.

Trump had planned to deliver a speech on Thursday in Denver about his immigration policy, but that event has been postponed, a campaign official confirmed.

On Sunday, Kellyanne Conway, Trump's campaign manager, was pressed repeatedly in a CNN interview on whether Trump would go ahead with a call he made in November to set up a "deportation force" to remove undocumented immigrants.

"To be determined," Conway said. She also said Trump was committed to a "fair and humane" approach to those living in the country illegally.

Trump's immigration position has been a centerpiece of his campaign. When he announced he was running for president, he called for construction of a wall along the nation's border with Mexico to stop undocumented immigrants from entering the country. He has insisted that he will make Mexico pay for the wall.

Trump also said that Mexico was sending "rapists" and "criminals" across the border.

(Reporting by Mohammad Zargham and Ginger Gibson; Editing by Alden Bentley and Jonathan Oatis)