According to the PMBOK (pg. 361), "Informal communications are general communications activities using emails, social media, websites, and informal ad hoc discussions."
The question context suggests that the meeting is "urgent" in nature. We can also gather from the context that there is no formal meeting agenda. Though the PM thought about certains topics and information that needs to be shared with the stakeholders, there was no formal meeting agenda sent out ahead of time.
Again, according to the PMBOK (pg. 102) in the "Manage Project Knowledge" process the differences between formal and informal communication is highlighted, "Formal, rigid communication requirements are good for sharing information. Informal communication is more effective for creating new knowledge and integrating knowledge across diverse stakeholder groups."
Again, given the context of the question, the PM will be using an informal communication technique because we are doing more than simply sharing a standard report (formal), but will be getting stakeholder feedback and having each group integrate the correction action and approve the correction action. This obviously leads to discussions on the best way forward, which is informal in nature.
Thus, we are using an informal verbal communication technique.