UN Secretary-General Antonio Guterres on Wednesday said that Israeli settlement activities in the occupied Palestinian territories have "no legal validity" and represent "a flagrant violation" of international law.

"Resolving the Israeli-Palestinian conflict remains one of the most intractable challenges facing the international community. Regrettably, over the past year, there have been no positive developments, and the situation on the ground continues to deteriorate," Guterres said in a letter marking the International Day of Solidarity with the Palestinian People.

The message was read out by his office chief Maria Luiza Ribeiro Viotti to a meeting held by the UN Committee on the Exercise of the Inalienable Rights of the Palestinian People to observe the international day.

The UN chief said the increasing pace of illegal settlements, demolitions of Palestinian homes and the calamitous conditions inflicted on Gaza must stop, adding that: "These actions threaten to undermine the viability of establishing a Palestinian state based on relevant United Nations resolutions."

He called on Israelis and Palestinians to opt for serious steps in order to restore faith in the two-state solution. "There is no viable alternative."

The International Day of Solidarity with the Palestinian People is an annual observance designated by the UN General Assembly on December 2, 1977, to commemorate its adoption of a resolution on November 29, 1947, to partition the mandate of Palestine into two states.