Ukrainian President Volodymyr Zelenskyy speaks in Kyiv, Ukraine. AP
The breakaway states of Donetsk and Lugansk, whose independence Moscow recognised in February, are situated in the Donbas region at the centre of Russia's invasion and have escaped Kyiv's control since 2014.
Syria provoked Ukraine's ire after becoming the first state other than Russia to recognise the two separatist republics earlier on Wednesday.
"There will no longer be relations between Ukraine and Syria," Zelenskyy said in a video posted on Telegram, adding that the sanctions pressure against Syria "will be even greater".
Zelenskyy described Syria's move as a "worthless story".
This is not the first time that the Syrian government, which since 2015 has been heavily backed by Russia in its own civil war, has supported Moscow's recognition of breakaway states.
In 2018, Syria recognised South Ossetia and Abkhazia as independent from the former Soviet state of Georgia, prompting Tbilisi to cut diplomatic ties.
Abkhazia and South Ossetia are internationally recognised as part of Georgia, which gained independence from the Soviet Union in 1991, but Russia and a handful of other countries recognise their independence.