Australia and New Zealand dispatched surveillance flights on Monday to assess the damage in Tonga, isolated from the rest of the world due to the eruption of an underwater volcano that triggered a tsunami and blanketed the Pacific island with ash.

Australian Prime Minister Scott Morrison pledged to provide support for Tonga as early as possible but said the volcano ash had hampered relief efforts.

"There have been a lot of challenges there with the ash cloud and the disruption to communications...