Republican Sen. Lindsey Graham, one of President Donald Trump's staunchest allies, issued a stern warning to Turkey over attacking Kurdish forces in northeastern Syria.

Turkey has expressed readiness to attack US-allied Kurdish forces in Syria who it considers an enemy following Trump's announcement that the US would withdraw its troops stationed there.

Graham on Tuesday vowed to impose "sanctions from hell" on Turkey if it moved troops into northeastern Syria.

Graham doubled down on his threats Tuesday night, stressing that Turkey did not have the go-ahead to move troops into the region and doing so would cross a "red line."

White House and Kurdish officials said they expected Turkey to launch a "major offensive" in northeastern Syria within 24 hours, Foreign Policy reported.

Republican Sen. Lindsey Graham, a consistent supporter of President Donald Trump, warned Turkey against invading northeastern Syria just days after Trump said the US would withdraw its troops there.

Graham on Tuesday vowed to impose "sanctions from hell" on Turkey if it moved troops into northeastern Syria. He also said Congress would call for Turkey's suspension from NATO if it were to attack the Kurdish forces who control the area.

Graham doubled down on his threats Tuesday night, stressing that Turkey did not have the go-ahead to move troops into the region and that doing so would cross a "red line."

"To the Turkish Government: You do NOT have a green light to enter into northern Syria," he tweeted Tuesday night. "There is massive bipartisan opposition in Congress, which you should see as a red line you should not cross."

Trump's decision to withdraw troops is widely seen as leaving the US's Kurdish allies in jeopardy and is paving the way for a major Turkish assault. The US has kept about 1,000 troops in Syria since the last Islamic State stronghold in the country, in the town of Baghuz, was defeated in March.

Republicans and former US officials slammed Trump's decision, saying the move was "shortsighted and irresponsible" and put America's Kurdish allies in danger.

Read more: Lindsey Graham, one of Trump's staunchest defenders, is slamming the president over his decision to pull out of northern Syria and says the White House is lying about ISIS

The Syrian Democratic Forces are the US's main allies in the region and have been fending off Islamic State militants for years. Led by Kurdish militias, the SDF has roughly 60,000 troops and occupies a medium-size area in northeastern Syria.

Turkey, however, lumps together the SDF and Kurdish forces with the Kurdistan Workers' Party, also known as the PKK, which has long fought an armed conflict for Kurdish independence against Turkey. The group has been listed as a terrorist organization by NATO, the US, the UK, Japan, and the European Union, and Turkey has expressed fury for years that the US has been arming Kurds.

Turkey is expected to launch a 'major offensive' in northeastern Syria

Turkish army howitzers positioned on the Turkish-Syrian border near Akcakale. Reuters

On Tuesday, Turkish officials told Reuters that they struck the Syrian-Iraqi border to prevent Kurdish forces from using the area to reinforce their personnel in the northeast.

Later on Tuesday, White House and Kurdish officials said they expected Turkey to launch a "major offensive" in northeastern Syria within 24 hours, Foreign Policy reported.

Bassam Saker, a representative of the political wing of the SDF, known as the Syrian Democratic Council, told Foreign Policy that State Department officials had told Kurdish leaders a Turkish attack would be met with "harsh economic and political sanctions."

Turkish President Recep Tayyip Erdogan told reporters on a flight to Serbia on Tuesday that an operation could occur "before the news could be printed," according to The New York Times.

President Donald Trump and Turkish President Recep Tayyip Erdogan at a NATO summit in Brussels. Reuters

A senior White House official told Foreign Policy that "significant numbers" of Kurdish fighters had moved toward the border to fend off an attack, leaving few to watch over prisons that hold Islamic State prisoners.

The SDF maintains control of tens of thousands of suspected ISIS members and their families, including about 70,000 women and children in a compound in the Syrian city of al-Hol, according to The Atlantic. Though ISIS has lost control of its urban territory, the group still has as many as 18,000 fighters quietly stationed across Iraq and Syria, according to The New York Times.

Read more: Here are the 5 major players that will feel the impact from Trump's decision to withdraw troops from Syria

The SDF on Tuesday said ISIS had taken advantage of the Kurdish movement and committed three suicide bombings on its positions in Raqqa.

Trump has said the US "may be" withdrawing its troops but had not abandoned the Kurds. On Tuesday, Trump threatened to "obliterate" the Turkish economy if Turkey acted in ways he did not condone.

"If Turkey does anything that I, in my great and unmatched wisdom, consider to be off limits, I will totally destroy and obliterate the Economy of Turkey (I've done before!)," he wrote on Twitter.