Clara Bensimon and Raymonde Mamane, sisters both in their 50s, are in hospital on respirators following the Nice terror outrage.
Ten Jews were wounded in the attack by an Islamist-inspired terrorist on Thursday night and four were still missing on Friday.
Chief Rabbi Ephraim Mirvis said that the prayers of British Jews were with the people of Nice.
More than 80 people were killed when a lorry ploughed through crowds celebrating the Bastille Day holiday.
Members of a Chabad day camp were close to the area at the time of the attack.
“We are praying for everyone who has been injured in this terrible attack,” said Chabad rabbi Yossef Yitzchak Pinson.
A representative of Nice's Consistoire, the umbrella body for local Jewish groups, said that security was very tight at Jewish buildings around the city, with most under 24-hour military protection.
"More than anything else, we feel solidarity with the families of those killed, and the people of Nice," she said.
Rabbi Mirvis said he was “shocked to hear about yet another incident bringing horrific loss of life to many in France.”
Rabbi Haim Korsia, France's Chief Rabbi, said that he sent his “sincere condolences to the families and relatives of the victims affected by the terrorism in Nice. My prayers and thoughts accompany you”.
British Jews were among those who escaped the carnage.
Joel Fenster, a former Noam Masorti Youth Mazkir and ex-Cambridge Union president, was in the area when the lorry ploughed through the crowds.
He later to spoke to the BBC to give his eyewitness account, and wrote on Facebook: “Thanks all for messages. Just got home. Safe now. Was metres from attack but spent last two hours hiding in alley then running home.”
The incident was similar to a number of attacks in Israel carried out by Palestinian terrorists in the past two years.
Shimon Peres, Israel’s former president, said: “Israel sends France its deepest condolences and stands united with our French friends in this difficult time.”
Meanwhile, Benjamin Netanyahu, Israel's Prime Minister, said: "Israel condemns in the strongest terms last night's horrific attack in Nice.
"We are again reminded that terrorism can strike anywhere and must be fought everywhere. Israelis stand united with the people of France today. We join you in mourning those killed and wish a speedy recovery to the wounded. Israel is ready to help the French government fight this evil until it is defeated."
Rabbi Alexandra Wright, co-chair of Liberal Judaism's Rabbinic Conference, said: "We extend our deepest sympathies to the people of France: to those who are bereaved, those who are wounded, whose losses will take years to heal, if at all.
"We mourn with those who mourn and pray for healing for those who have been hurt. And we stand with the people of France and all those who suffer at the hands of the tyranny of terrorism."
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