Turkey intensified its air and artillery strikes in northeast Syria on Friday, escalating an offensive against the Kurdish militia that has drawn warnings of a humanitarian catastrophe.

U.S. lawmakers introduced a resolution on Friday opposing U.S. President Donald Trump's decision to clear the way for Turkey to attack Kurdish fighters.

Lawmakers in both the Senate and House of Representatives are also working on legislation that would impose stiff sanctions on Turkey and its leaders over its offensive.

Trump's defense secretary, Mark Esper, told a briefing that the United States had not abandoned the Kurds. U.S. officials had urged Turkey to halt the assault, Esper said, warning of "dramatic harm" to bilateral relations.

U.S. Treasury Secretary Steven Mnuchin said on Friday that Trump had authorized U.S. officials to draft "very significant" new sanctions to target Turkey.

In response, Turkey's Foreign Ministry said late on Friday that Turkey would retaliate against any steps against its efforts to fight terrorism.

Turkish forces have seized nine villages near Ras al-Ain and Tel Abyad, said Rami Abdulrahman, director of the Syrian Observatory for Human Rights, which monitors the war.

The U.S. officials said that an explosion that may have been caused by Turkish artillery struck near a U.S. military outpost in northern Syria on Friday, but no U.S. personnel were reported hurt while Turkey's Defense Ministry said there was no firing on the U.S. observation post.

Meanwhile, the Turkish president threatened to send 3.6 million refugees to Europe if the European Union did not back his assault.

"We will never accept that refugees are weaponized and used to blackmail us," European Council President Donald Tusk wrote on Twitter. France said sanctions against Turkey would be discussed at an EU summit next week.

Read more:

No consensus at UN Security Council as Syria crisis deepens

French President Emmanuel Macron's office said that France and the United States have agreed to remain in close contact over Turkey's escalating campaign in northern Syria, adding that he had stressed the need to try to end the offensive in a phone call with Trump late on Friday.

A war monitor gave a death toll of more than 100 from the first days of the assault. The United Nations said 100,000 people had fled their homes.