John McDonnell failed to condemn Hamas' deadly attacks on Israel as he addressed a Palestine Solidarity Campaign at the Labour Party Conference.
Speaking at a fringe event in Liverpool, the former shadow chancellor said that he was "worried" about the situation unfolding in the Middle East and the impact on civilians.
However, he failed to address the terror attacks launch by Hamas on Israel over the weekend.
The Hayes and Harlington MP said the focus now needed to be on sorting a ceasefire.
WATCH: John McDonnell on Hamas
He said: "I think our role as individuals and members of the Labour and trade union movement, and every other organisation that we can possibly mobilise as well, is to try and get some acknowledgement that the key factor now must be the protection of civilians.
"I’m so fearful, I’m so worried that what’s happening in Gaza now is that we’re seeing almost an annihilation of civilians.
"Therefore, everything we have to do now is to mobilise both within our own country, but internationally as well, to try and put pressure on the international community now to ensure that the attacks on Gaza end and that a form of dialogue is opened up to secure initially, at least, some form of ceasefire, de-escalation, and then hopefully the first steps to a longer-term solution.
"And I know we’ve been here so often before but I’m not sure we’ve been here on this scale of the potential loss of life, civilian loss of life in particular.
More than 1,000 people died in Saturday's terror attack
"If this continues on in this form, it could set the Middle East alight again on a scale we’ve not seen, maybe ever."
More than 1,000 people died when Israel was targeted on Saturday, with hundreds of Palestinians having since been killed in retaliatory strikes.
Despite failing to condemn Hamas' actions during the Palestine Solidarity Campaign event, when arriving for the panel discussion, McDonnell said that he had released a statement criticising their attack yesterday.
He refused to answer when asked by reporters if he condemned Hamas as an organisation.
Last night McDonnell went further than his old ally Jeremy Corbyn, the former Labour leader, in proactively saying he would “condemn the killing of innocents” by Hamas.
“But as a father and a grandfather, my heart also goes out to the parents of the children killed in Gaza,” the former shadow chancellor added at an event chaired by a Unite director.
“There is no justification for the killing of civilians on any side.
“My fear now is with the Israeli bombings and the threat of ground invasion of Gaza and the blockage today of water, electricity and food supplies.
“I fear for the Palestinian parents who now see their children living in a nightmare situation of wondering what will happen next.”