Trump spoke with Erdogan over the phone late Monday during which both leaders discussed cooperation on Syria and anti-terrorism, the source, who spoke on condition of anonymity due to restrictions on talking to the media, said.

The two leaders discussed the Assad regime's chemical attack on April 4 that killed around 100 civilians and injured 500 others in the opposition-held town of Khan Sheikhun in Idlib province.

Investigators in Turkey and international bodies believe sarin gas was used in the attack.

“We cannot stay silent over regime attacks anymore,” Trump told Erdogan, the source said.

They agreed Bashar al-Assad was responsible for the attack.

The American president thanked Turkey for backing the U.S. missile strikes on an Assad regime air base in Syria in retaliation for the chemical attack. Both sides highlighted the need for cooperation in the fight against terrorist groups, including Daesh, the source said.

Trump also told Erdogan he cared about the friendship between Turkey and the U. S., and noted there were important things to do together.

The two leaders also agreed to increase bilateral cooperation and partnership in Syria and Iraq, and the fight against the PKK, the source added.

Trump also congratulated Erdogan over the Yes win in the constitutional referendum on Sunday.

The call came a day after more than 51 percent of Turks voted in favor of 18 constitutional amendments that will, among other things, see Turkey switch from a parliamentary to a presidential system of governance.

Anadolu Agency