"Dozens of tanks, armored vehicles and personnel carriers, as well as hundreds of Yemeni soldiers trained in Saudi Arabia, arrived in Yemen overnight" via the Wadia border post in the north of the country, a Yemeni military source told AFP.
"These military reinforcements came from Saudi Arabia's Sharura region and are intended for the popular resistance and the national army," another military source said, referring to forces loyal to Yemeni President Abedrabbo Mansour Hadi, currently in exile in Riyadh.
Since March 26, a Saudi-led military coalition has supported the loyalists with airstrikes to stop the advance of the Huthi rebels, who last year took over the capital Sanaa and pressed south into second city Aden earlier this year.
Tribal sources told AFP that the reinforcements were headed towards the provinces of Marib, east of Sanaa, and Shabwa, to the southeast, "to expel the Huthis and their allies" from these two provinces, where heavy fighting has been ongoing.
Pro-Hadi forces continue to gain ground in the south of the country after retaking Aden last month and seizing the country's largest airbase of Al-Anad to its north on Tuesday.
This turnaround in the fighting coincided with the appearance on the battleground of modern military equipment that, according to military sources, the Saudi-led coalition had provided to Hadi's supporters.
A military source on Monday reported the presence of "hundreds of soldiers from Gulf countries" that were members of the coalition in Aden, where they landed with "dozens of tanks and armored vehicles" to "secure the city."
Also on Thursday, pro-government forces and the rebels carried out a prisoner exchange, said the International Committee of the Red Cross, which monitored the operation.
A Red Cross statement said seven Huthi rebels were transferred from Aden to Sanaa.
They boarded a plane to Sanaa in return for the release of more than 20 southern pro-government fighters, the head of security at Aden airport, Col. Ahmed al-Dulahi, told AFP.
The ICRC also said that its president, Peter Maurer, would visit Yemen Aug. 8-10 "to highlight the dire humanitarian situation in the country."
The war in Yemen has killed nearly 4,000 people, half of them civilians, while 80 percent of the 21 million population needs aid and protection, the UN says.