US President Donald Trump has lauded his relationship with Turkish President Recep Tayyip Erdogan after the two leaders met in Washington DC.

There have been mounting differences between the two NATO allies ranging from Syria policy to Turkey's purchase of a Russian missile defense system.

Mr Trump's warm welcome of the Turkish president comes amid anger in the US Congress about Turkey's offensive into Syria to drive out a Kurdish militia, the US's main partner in the fight against the Islamic State group.

"We've been friends for a long time, almost from Day 1. We understand each others' country. We understand where we are coming from," Mr Trump told Mr Erdogan as they sat next to each other in the Oval Office.

 "They're highly respected in their country and in the region," Mr Trump said of Mr Erdogan and his wife, Emine.

In front of the White House, protesters denounced Mr Erdogan's visit and urged Mr Trump to protect Kurds threatened by Turkey's incursion in Syria.

 One sign read, "America Stand With Your Kurdish Allies."

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Mr Trump also said they would discuss Turkey's purchase of a Russian S-400 missile defense system. He also added that the two sides would talk about a potential $100 billion trade deal.

"We're also talking about the trade deal...Frankly, we're going to be expanding our trade relationship very significantly," Mr Trump said.

Despite his warm welcome, the two NATO allies have been at loggerheads for months now and their ties hit a new crisis point last month when Mr Erdogan began a cross-border incursion against America's Kurdish allies in Syria and upended the US presence there.

The US has also been livid over Turkey's purchase of Russian missile defense systems.

Turkey shrugged off threats of US sanctions and began receiving its first S-400 deliveries in July.

In response, the US removed Turkey from the F-35 fighter jet program, in which Turkey was a manufacturer and buyer.

The US has not imposed any sanctions so far.

Five Republican senators, including vocal Turkey critic Lindsey Graham, came to the White House and spoke with the two leaders about the F-35 program.

"That's what we’re here for, we’re talking about with our great senators. There are a lot of alternatives. We’ll work something out. I project that we will work something out," Mr Trump said.

Turkey's S-400 purchase infuriated the US Congress.

The House of Representatives last month passed a sanctions package to punish Turkey over its Syria operation which key members of the Senate, such as Mr Graham – a Trump ally - have vowed to advance if Turkey endangers Kurds.

Republican Senator Ted Cruz pressed Mr Erdogan on Turkey'st reatment of Syria's Kurds.

"The Kurds have risked a lot to stand with America and fightour shared enemy and there is very real concern that we do not want to see Turkey engaged in offensive actions against the Kurds," Mr Cruz said.