WASHINGTON (REUTERS) - US Republican presidential nominee Donald Trump tweeted on Sunday night (Monday morning, Singapore time) that he would make a major speech on illegal immigration in Arizona on Wednesday (Aug 31).

Mr Trump said on Saturday that he would crack down on illegal immigrants who overstay their visas as he sought to clarify his views on how to overhaul the US immigration system.

The comment came after he said earlier last week that he was softening on his plan to deport all 11 million illegal immigrants already in the US.

That stance had aroused criticism from conservatives who wanted him to stand fast after he won the Republican presidential nomination in large part with a hard-line position that called for building a wall along the US border with Mexico.

"I will be making a major speech on ILLEGAL IMMIGRATION on Wednesday in the GREAT State of Arizona. Big crowds, looking for a larger venue," Mr Trump said in his tweet. He had postponed an immigration speech originally planned for last Thursday in Denver,

In another tweet on Sunday night, Mr Trump said both he and Hillary Clinton, his Democratic rival for the Nov 8 election, should release detailed medical records. "I have no problem in doing so! Hillary?"

In his speech on Saturday in Iowa, Mr Trump said he would seek to institute a tracking system to ensure that illegal immigrants who overstay their visas are quickly removed, and would propose an e-verify system to prevent undocumented residents from gaining access to welfare and other benefits.

"If we don't enforce visa expiration dates, then we have an open border - it's as simple as that," he said.

Mr Trump said his first priority on taking office next January would be the immediate deportation of thousands of undocumented immigrants who remain in the United States despite having committed crimes.

"These international gangs and cartels will be a thing of the past," he said. "Their reign of terror will be over. In this task, we will always err on the side of protecting the American people - we will use immigration law to prevent crimes, and will not wait until some innocent American has been harmed or killed before taking action."

He did not explain how his plan would affect undocumented residents who have been in the United States for decades and obeyed US laws.