The two US companies are, respectively, the second and third largest airlines in the world.
United cited “security reasons” while Delta said it is “continuously monitoring the evolving security environment.”
FlyDubai and Brussels Airlines have also announced cancellations.
Some now fear other firms will follow their lead.
On Thursday morning, a Lufthansa flight en route to Tel Aviv landed in Lanarca, Cyprus before returning to Germany.
While the airline initially told passengers the plane was stopping short of Israel due to "technical reasons", Channel 12 reported the decision was made because the crew refused to fly any further.
Lufthansa told the broadcaster the plane landed in Cyprus “as a precautionary measure due to security activity.”
The decisions to suspend flights to Israel come in the wake of the assassination of Hamas leader Ismail Haniyeh in Tehran and senior Hezbollah military commander Fuad Shukr in Beirut.
As Iran considers how to respond, Western governments have advised their citizens to leave Lebanon amid fears of a regional war.
Analysts expect that a large-scale attack against Israel may come in the next few days.
