More than 72,000 migrants irregularly crossed from Morocco into the tiny Spanish territory between July 30 and 31 in an unprecedented surge at one of the European Union's only two land borders with Africa, prompting alarm across the bloc.
At least 100 people died in the attempt, according to Ceuta's local authorities.
Spanish opposition parties and some human rights groups have accused Morocco of deliberately allowing the mass crossing, an accusation the country's authorities have denied.
"When people talk and speculate about the participation of Moroccan authorities, I deny this outright," Sanchez told radio station SER.
Videos on Instagram, TikTok and other social media spread misleading claims that migrants reaching Ceuta by sea could remain in Spain, encouraging thousands to attempt the crossing, Spanish authorities have said.
Sanchez said research by the EU's diplomatic service European External Action Service (EEAS) had indicated that Russian and Israeli networks had been behind the spread of disinformation in conjunction with ultranationalist networks.
He said it was clear that the two countries had used the crisis to criticise Spain due to dissatisfaction with its position on the wars in Ukraine and the Gaza Strip but gave no further details on the evidence he had of their involvement.
Later on Monday, Israeli Foreign Minister Gideon Sa'ar said in a post on X that Sanchez was "lying shamelessly" when accusing Israel of disseminating misleading information about Ceuta, casting it as an attempt to distract from domestic policy failures.
The Russian government rejected Spain's allegations that it was connected to the Ceuta migration crisis, Kremlin spokesman Dmitry Peskov said on Monday.
The EEAS did not immediately respond to Reuters' requests for comment.
About 5000 migrants, some seeking asylum, have remained in Ceuta since the border rush, triggering near-daily protests from locals against the Spanish government's handling of the crisis and sporadic attacks against a makeshift migrant camp at El Trampolin beach, according to police reports.
Police arrested three Moroccan citizens on Sunday for lobbing bottles filled with corrosive liquids against an army patrol in Ceuta, news agency EFE reported, citing police sources.
No one was injured in the attack, the report added.
Sanchez also announced on Monday that the cabinet would deploy an additional 168 million euro ($A272 million) in emergency funds to reactivate Ceuta's economy, a figure equivalent to 8 per cent of the enclave's gross domestic product, on top of 180 million euro pledged last week for medium-term recovery efforts.
