President Donald Trump called Turkish President Recep Tayyip Erdogan to congratulate him on his referendum victory, a controversial election that increased his governing powers, the White House said Monday.

The referendum greatly expanded Erdogan’s power and limited the country’s parliament’s. Turkey’s opposition party has vowed to challenge the results.

International observers called the validity of Erdogan’s Sunday victory into question, citing the counting procedures used in the referendum and acknowledging that the vote took place on an “unlevel playing field.” Erdogan has rejected these critiques, claiming the vote was “the most democratic election” in any Western country.

Earlier Monday, the U.S. State Department released a statement thanking international observers for their preliminary assessment of the report, and mentioning reports of “irregularities” during the referendum vote.

Read More: Erdogan’s Victory in the Referendum on His Powers Will Leave Turkey Even More Divided

While the White House acknowledged Trump congratulated Erdogan, the statement also emphasized that the two leaders discussed the ongoing civil war in Syria.

“President Donald J. Trump spoke today with President Recep Tayyip Erdogan of Turkey to congratulate him on his recent referendum victory and to discuss the United States’ action in response to the Syrian regime’s use of chemical weapons on April 4th,” the White House said in a statement. “President Trump thanked President Erdogan for supporting this action by the United States, and the leaders agreed on the importance of holding Syrian President Bashar al-Assad accountable.”

Get The Brief. Sign up to receive the top stories you need to know right now. Please enter a valid email address. Sign Up Now Check the box if you do not wish to receive promotional offers via email from TIME. You can unsubscribe at any time. By signing up you are agreeing to our Terms of Use and Privacy Policy . This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply. Thank you! For your security, we've sent a confirmation email to the address you entered. Click the link to confirm your subscription and begin receiving our newsletters. If you don't get the confirmation within 10 minutes, please check your spam folder.

Write to Alana Abramson at Alana.Abramson@time.com.