The official Moroccan news agency MAP's latest provisional toll stood at 229, with 120 injured, in the pre-dawn quake in Al Hoceima Province.
But a local official reached in the town of Imzouren estimated that at least 300 people were killed, adding about 40 houses were "entirely destroyed" as residents were sleeping when the quake struck at 2:27 am (0227 GMT).
Search and rescue teams were finding more victims as they reached many small villages in the remote mountainous Mediterranean region, he said, as an AFP journalist saw scenes of devastation on arriving at Imzouren, some 10 kilometres south of Al Hoceima port.
Rescue teams rushed to the scene after the quake, which was felt as far away as parts of southern Spain and estimated at 6.3 on the Richter scale by the seismic officials in the French city of Strasbourg.
Morocco's King Mohammed VI was in the region late on Tuesday to visit the quake-hit site "to be close to his subjects and to follow the rescue operations", MAP agency said.
Many shocked and injured people had flooded into Al Hoceima in a state of "total panic", doctor Mohammed Lacghar told AFP by telephone, saying he had "never seen anything like it" in the clinic he ran in the town.
Local officials said hundreds of injured were coming to the port of 100,000 residents, which seemed to have escaped damage from the quake.
One hospital in Al Hoceima was overflowing and the injured were being evacuated to a local army barracks and to a charity home, Mohamed Boudra, head of the local council, said.
Armed forces joined the royal gendarmerie, or paramilitary police, as well as local rescue services in the search, MAP said.
Helicopters were also flying in large supplies of search and rescue equipment, MAP said.
"We recorded hundreds of aftershocks of this earthquake, a lot weaker than the initial tremor," Jabbour Nasser of the Moroccan Geophysical Laboratory said on television.
The Mohammed V Foundation provided two planes and a medical team as well as ambulances, medicines and equipment including blankets and 20 tonnes of food, officials said.
United Nations Secretary General Kofi Annan offered UN help and was, according to a statement, "deeply saddened by the heavy loss of life and the extensive damage caused".
European countries swiftly pledged help, with France sending in experts to assess needs, and putting a team of 60 other rescue workers including doctors, firemen and dog handlers on standby.
Belgium said it was prepared to send aid if requested and offered condolences, as did Germany.
"The federal government is prepared at any time to supply aid and support to alleviate the suffering," said Foreign Minister Joschka Fischer in a telegram to his Moroccan counterpart Mohamed Benaissa.
King Juan Carlos of Spain telephoned Morocco's King Mohammed VI to express "the solidarity of the Spanish people", Spain's royal palace said. Spanish Prime Minister Jose Maria Aznar also sent his condolences and offered aid.
But Moroccans have criticised the Spanish enclave of Melilla - on the Mediterranean coast, 250 kilometres (155 miles) west of the quake-hit province - for not being forthcoming with aid, a local official said.
Inhabitants of the devastated region "would have liked Melilla, which has strong medical resources and rescue equipment, to come and help as soon as the tragedy was known about", said Ilias Al-Omari, an official in Al Hoceima.
People believe the "catastrophe would have been less, especially for the injured", he added.
The tremor was felt in several parts of Morocco, including the regions of Fez and Taza further to the south as well as Melilla and the southern Spanish regions of Andalusia and Murcia.