WASHINGTON, Nov. 13 -- U.S. President Donald Trump and his Turkish counterpart Recep Tayyip Erdogan failed to reach an agreement on major bilateral differences during their Wednesday's meeting at the White House.

In a joint press conference after the meeting, Trump said Turkey's purchasing of the S-400 missile system from Russia "creates some very serious challenges."

"We are talking about it constantly, we talked about it today, we're talking about it in the future," Trump said, indicating that the United States failed to persuade Turkey to abandon its purchasing of the system.

Trump stressed his good relationship with Kurdish forces in Syria, which are viewed as a threat by Ankara.

Erdogan also said the two sides were unable to reach an agreement but pledged to carry out further dialogue.

During the meeting, which was also attended by U.S. lawmakers, Republican Senator Ted Cruz challenged Erdogan on the issue of S-400 and Turkey's Kurdish policy.

"We very much hope Turkey changes the path it's on and does not employ the S-400," Cruz said. "We do not want to see Turkey engaged in offensive action against the Kurds."

"I'm hoping to salvage this relationship, but only time will tell if that is possible," U.S. Senator Lindsey Graham said in a statement after meeting with Erdogan.

Before Erdogan's visit, U.S. State Department officials acknowledged that the two countries were "at loggerheads" on many issues.

This is Erdogan's second visit to the White House after Trump assumed the presidency in 2017.