Regional states must show unity to counter Iran's regional interference, Saudi Arabia's Crown Prince Mohammed bin Salman said at the GCC summit on Tuesday.
The call came amid signs that a years-long rift with Qatar had been resolved.
The Saudi crown prince and Qatar's Emir Sheikh Tamim exchanged a warm embrace on the tarmac at Al Ula airport after the Qatari delegation arrived for the 41st Gulf Co-operation Council summit.
Qatar's emir is attending the summit for the first time since 2017, when fellow bloc members Saudi Arabia, the UAE and Bahrain, along with Egypt, cut off ties over Doha's support for extremist groups.
"We are today in need of co-operation in our efforts, in the face of challenges in our region – especially the nuclear and ballistic missiles programme in Iran, which aims to shake up the stability of the region," Prince Mohammed said in the opening address.
“We need to realise the aims that the GCC is based on, the continued journey in all fields,” he said.
He thanked the gathered leaders for their "bold steps" in attending the summit.
Referring to the summit statement signed by all six GCC leaders, Prince Mohammed said: "The Al Ula Accord emphasises Gulf, Arab and Islamic solidarity and stability as well as the strengthening of friendship and brotherhood between our countries and people."
The contents of the accord were not immediately made public.
Speaking after Prince Salman, Kuwaiti emir Sheikh Nawaf Al Sabah said the Al Ula Accord "can be called the solidarity agreement".
"We congratulate everyone on the historic achievement that has been reached here at the summit," he said.
Sheikh Nawaf was attending his first summit as Kuwaiti emir after succeeding his brother Sheikh Sabah, who died in September.
Prince Mohammed said the summit would be named after Sheikh Sabah and Sultan Qaboos of Oman, who died in January last year.
"This year, the GCC summit is missing two great leaders who played a major role in mediating talks between member states, the late Sheikh Sabah Al Ahmad Al Sabah and the late Sultan Qaboos," Prince Mohammed said in his speech.
The summit was held amid expectations that the rift with Qatar would be resolved following Saudi Arabia's announcement on Monday evening that it was reopening land, sea and air borders with its neighbour, which had been closed since 2017.
Egypt's Foreign Minister Sameh Shukry also attended the summit.
“Egypt appreciates and values every sincere effort made to achieve reconciliation between the four nations [Saudi Arabia, UAE, Bahrain and Egypt] and Qatar, especially the efforts of the brotherly state of Kuwait over the past years,” said the Foreign Ministry statement.