BRUSSELS - U.S. President Joe Biden and Turkish President Tayyip Erdogan will on Monday discuss strained relations between the two NATO allies in their first face-to-face meeting since Biden took office, but there is little hope for a major breakthrough.

Turkey has the Western alliance's second-biggest military after the United States but Ankara's purchase of Russian defense systems, disagreements over Syria, human rights, the treatment of Armenians in the Ottoman Empire and tensions in the eastern Mediterranean have hurt ties.

The most challenging problem has been Turkey's purchase of the Russian S-400 systems, a rift that also divides NATO. Washington has already removed Ankara from the F-35 fighter jet program and imposed sanctions over the acquisition.

As president, Biden has recognised the 1915 massacre of Armenians as genocide and stepped up criticism of Turkey's human rights record, adopting a cooler tone towards Erdogan than his predecessor Donald Trump.

White House national security adviser Jake Sullivan said the two leaders would also touch on Afghanistan, Libya and China.

"They will talk about political and economic issues that are more challenging ... that have been challenges in our relationship, including human rights-related issues," Sullivan told reporters on Sunday.

One area where Erdogan hopes to showcase a central Turkish role in NATO is Afghanistan, where Turkey has offered to guard and operate Kabul airport to secure access to the country after the U.S.-prompted NATO withdrawal.

At the start of the main leaders' session at NATO, Biden spoke to Erdogan at length in a small group before they took their seats. He is expected to hold a news conference at 1800 GMT.

Leaving Turkey, Erdogan said he expected an "unconditional approach" from Biden to moving on from past troubles.

"Turkey is not just any country, it is an allied country. We are two NATO allies," he told reporters.

"There are many issues regarding defence industry that were left on the table. The most important one of these is the F-35 issue," Erdogan said.

Before the summit, Erdogan met French President Emmanuel Macron. Ankara and Paris have been at odds over Syria, Libya, and Turkish criticism of the fight against what Macron calls Islamist separatism, among other issues.

Macron and Erdogan discussed working together on Libya and Syria while the French president said his country and its secular laws respected all religions, including Islam.

But with so many points of contention, hopes for any substantial breakthrough are slim.

"While both sides are expected to pursue some pragmatic engagement and neither is interested in a showdown, the meeting will not provide Erdogan with any meaningful dividend," consultancy Teneo said ahead of the talks.

(Additional reporting by Jonathan Spicer and Steve Holland, Writing by Gabriela Baczynska and Humeyra Pamuk, Editing by Catherine Evans and Giles Elgood) ((gabriela.baczynska@thomsonreuters.com; +32 2 287 68 39; Reuters Messaging: gabriela.baczynska.thomsonreuters.com@reuters.net))