The WikiLeaks founder has normally had internet and social media access at the embassy, where Ecuador granted him asylum and allowed him to stay continuously for nearly six years to avoid an arrest warrant.
But Ecuador says it cut off his outside communications access -- for at least the second time in two years -- on Tuesday.
"The government of Ecuador has suspended all systems that allow Julian Assange to communicate outside the Ecuadorian Embassy in London," Ecuador's statement, released Wednesday, reads.
"This decision was taken due to Assange's failure to commit to the written agreement he agreed with the government at the end of 2017, where he was obligated not to release any messages that would interfere with other countries' matters.
"Ecuador's government warns that Assange's behavior through his social media messages puts in risk the good relationship the country has with the UK, other EU countries and other nations."
The statement did not specify which message or messages, if any, violated the agreement.
WikiLeaks denied on Twitter that Assange had agreed to a deal with the Ecuadorian government.
"Claims made by Ecuador's public affairs office that @wikileaks editor @julianassange, arguably the world's best known free speech avtivist (sic), is under a gag agreement, are, perhaps unsurpringly (sic), entirely false.," the organization said on its verified twitter account.
Assange has been holed up at the Ecuadorian Embassy in London since 2012 in an effort to avoid a Swedish arrest warrant for alleged rape -- accusations he has always denied.
Sweden dropped the charges in May 2017, but Assange remains the subject of a UK arrest warrant.
Assange has recently tweeted about a wide range of subjects, including the UK's accusation that Russia was likely responsible for a nerve agent attack on a former Russian spy in Britain.
"While it is reasonable for (UK Prime Minister) Theresa May to view the Russian state as the leading suspect, so far the evidence is circumstantial & the OCPW (Organization for the Prohibition of Chemical Weapons) has not yet made any independent confirmation, permitting the Kremlin (to) push the view domestically that Russia is persecuted," Assange posted to Twitter on Monday.