© ZvezdaOSCE observers at the Russian Gukovo checkpoint.
The first shell exploded at Gukovo at around 13:55 Moscow time (0955 GMT), Russian border guard spokesman Vasily Malaev told Itar-Tass news agency.
"During this time, there were not only customs officers and border guards, but also representatives of the OSCE (Organization for Security and Co-operation in Europe) mission present at the crossing," Malaev stressed, adding that no one was hurt as a result of the incident.
Gukovo was hit by a mortar shell, which left a "the crater with the diameter of 40 centimeters and depth of 20 centimeters," said local customs spokesman Rayan Farukshin.
This spot is most likely "sighted" by those, who are executing fire, as Russian border guards went under gun fire there on Friday as they tried to document the damage done by an earlier shelling, he stressed.
The OSCE has confirmed that Gukovo checkpoint in Russia's southern Rostov Region was shelled on Saturday.
"It's true," Shiv Sharma, OSCE spokesman, told Itar-Tass news agency. "Our staff heard the sound of artillery fire or something like that. The incident didn't interrupt the work of our representatives, who remain at the crossing."
OSCE Permanent Council decided to send a monitoring mission to the Russian-Ukrainian border on July 24.
In accordance with the mandate, civilian observers are placed at Gukovo and Donetsk crossings during the next three months.
Gukovo checkpoint was shelled again later on Sunday, with the second incident occurring several hours later.
"At Around 17:30 Moscow time (1330 GMT), another projectile exploded at the crossing," Malaev said. "As a result, the border guard HQ was damaged. The nature of the damage will be established later."
Journalists from Russia's Zvezda channel, who were at the crossing during the second incident, said that it was "very scary."
"At first, explosions were heard in the distance," Vasily Kuchushev said as he appeared on air at Zvezda channel. "But then we heard a huge blast and saw a flash in the sky, with debris flying in all directions."
Russian border checkpoints in the Rostov Region were repeatedly shelled from the Ukrainian side during summer.
In mid-July, explosive shells were also fired at the Russian town of Donetsk - a namesake of the militia-held city of Donetsk in Ukraine. Back then, two shells hit residential areas, killing one person.
The Rostov Region is the main hub for Ukrainian refugees, who are fleeing for Russia in order to escape the bloody conflict in the country.
The government's crackdown on the south-east started in mid-April, after people in the Donetsk and Lugansk regions refused to recognize the coup-imposed authorities in Kiev and demanded federalization.
The Ukrainian military and National Guard resorted to airstrikes and shelling in their struggle against the self-defense forces in Donetsk and Lugansk.
Some 1,129 people have been killed and nearly 3,500 wounded in eastern Ukrainian violence, the UN announced in late July.
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