Twitter and the Huffington Post UK confirmed they were hacked after the Syrian Electronic Army made claims it had taken over the sites.
Twitter said the hack led to availability issues for an hour and a half but that no user information was compromised.
The Huffington Post attack was limited to the blogging platform's UK web address, a spokesperson said.
The attacks came as the Obama administration considers taking action against the Syrian government.
More top news
The cousin-in-law of Mexican footballer Alan Pulido - kidnapped and rescued over the weekend - is suspected of playing a role.
Thousands of US-backed troops in Syria are launching an attack to capture a crucial swathe of northern Syria from Islamic State.
A father who forced his pregnant 14-year-old daughter to marry a 24-year-old family friend who raped her has been jailed.