The United States on Monday imposed new sanctions targeting Syria's oil sector, lawmakers and intelligence officers, vowing no let-up in pressure on President Bashar al-Assad despite his gains on the ground.

"The Assad regime has a choice: take irreversible steps toward a peaceful resolution of this nearly decade-long conflict or face further crippling sanctions," Secretary of State Mike Pompeo said in a statement.

The Treasury Department and State Department barred transactions with 19 individuals or entities and froze any US assets they might have -- its fifth round of sanctions since a tough new US law on Syria came into force.

Sanctioned entities include two partners of Syria's petroleum ministry -- Arfada Petroleum Private Joint Stock Co and Sallizar Shipping, which are at work on an oil refinery in war-ravaged Raqa province and a terminal in the coastal city of Tartus.

The United States also slapped sanctions on General Ghassan Jaoudat Ismail, head of Syrian Air Force Intelligence, and Brigadier General Nasr Al-Ali, who heads the Political Security Directorate, a spy agency in charge of curbing dissent against Assad.

The Caesar Act, which took effect in June, restricts any US reconstruction assistance and keeps up pressure on Assad, demanding accountability, even though he has won back control of most of Syria following more than nine years of bloodshed.

The State Department said it was imposing the latest sanctions in memory of the more than 70 civilians killed in an October 2015 bombing of a marketplace in Douma, a city near Damascus then under rebel control that was hit three years later by a chemical attack, according to a UN probe.

Six days after US elections in which Democrat Joe Biden beat President Donald Trump, Pompeo announced that the US point man on Syria, James Jeffrey, 74, would retire this month.

A former US ambassador to Baghdad and Ankara, the Turkish-speaking diplomat has been a key go-between with President Recep Tayyip Erdogan, who last year assaulted US-allied Syrian Kurds following a pullout ordered by Trump.

Nathan Sales, the top State Department counterterrorism official, will take over both as the US representative on Syria and envoy to the coalition to defeat the Islamic State group, Pompeo said.