Tripoli, Libya (CNN) -- Explosions erupted in Tripoli for a second consecutive night Wednesday into Thursday morning as NATO jets struck a vocational secondary school, a Libyan government official said.
"We're not in a position to independently confirm that allegation," a NATO official said.
The attacks got under way hours after a Pentagon spokesman acknowledged that the United States is providing munitions for allies to use in the air campaign against Libya.
"We have provided material support, including munitions, to allies and partners engaged in operations in Libya," Col. Dave Lapan said in a statement.
But, as President Barack Obama warned Libyan leader Moammar Gadhafi on Wednesday that there would be "no letup" in NATO airstrikes, Russia lashed out against the air campaign, calling the bombing of certain Tripoli targets a "flagrant deviation" from the United Nations mandate.
Obama makes the case for Western leadership
U.N. Secretary-General Ban Ki-moon said Wednesday he had spoken with the Libyan prime minister and stressed the urgent need for a cease-fire.
The U.N. chief said the special U.N. envoy for Libya, who has been in Benghazi meeting with the Libyan opposition, would return soon to Tripoli to try to secure an agreement.
The war has dragged on for more than two months, with reports of horrific fighting and civilian casualties.
The Russian Foreign Ministry alleged Wednesday that several "non-military facilities in Tripoli that had already been bombed, were again attacked in a massive NATO airstrike, leading to civilian casualties." It said in a written statement that "this is yet another flagrant deviation" from the U.N. Security Council resolution 1973.
That resolution authorized all means necessary, short of occupying forces, to enforce a no-fly zone and protect Libyan civilians from attacks by forces loyal to Gadhafi.
A barrage of at least 18 rockets struck Tripoli Tuesday, killing 19 people and wounding another 150 in the heaviest onslaught since NATO's aerial strikes began, a government official told CNN.
NATO said it had struck a government vehicle storage facility adjacent to Gadhafi's compound.
The Russian ministry said NATO has justified its attacks by citing the need to protect civilians but said that it was "impossible to deny the obvious fact that airstrikes are not stopping military confrontation between the warring Libyan sides, and are only causing additional suffering of peaceful Libyans."
It said the airstrikes are failing to end the Libyan conflict, which was triggered by protests in February and an ensuing crackdown by Gadhafi's forces.
Russia, which has been critical of the airstrikes all along, has said normalization in Libya can come only from an immediate halt in military activities, the ministry said.
Some have viewed the Libya campaign as a litmus test for relations between the alliance and Russia. The 28 members of NATO work with Russia as equal partners in the NATO-Russia Council, a group that works on a wide array of issues, including security.
Obama, who met Wednesday in London with his British counterpart, David Cameron, increased pressure on Gadhafi by repeating that, ultimately, the embattled strongman must go.
The mission to protect Libyans from their leader means making sure Gadhafi "doesn't have capacity to send in a bunch of thugs to murder innocent civilians and threaten them," Obama said.
"I do think we have made enormous progress in Libya. We have saved lives. Gadhafi and his regime need to understand there will not be a letup in the pressure we are applying," Obama said.
But he and Cameron said it will take time to reach a solution in the North African country, ruled with an iron fist by Gadhafi for 42 years.
"We may have to be more patient than people would like," Obama said of the apparent stalemate.
In an effort to break the monthslong standoff, South Africa President Jacob Zuma plans to visit Tripoli next week for talks with Gadhafi, according to a statement on the South African leader's website.
Zuma will meet Gadhafi Monday in "his capacity as a member of the African Union high-level panel for the resolution of the conflict in Libya," the statement said. The panel also includes Uganda and Mauritania.
It is the second such trip for Zuma, who was part of an African Union delegation that visited the nation last month. That visit, which included talks on a cease-fire, did not succeed.
The Libyan government lashed out Wednesday at Obama and Cameron.
"It is illegal and illegitimate to speak about regime change under the pretext of protecting civilians," said a government official. "It is up to the Libyan people to decide when and if Colonel Gadhafi should go -- not the U.S. and the U.K."
Russia's official RIA Novosti news agency reported that Gadhafi's government may ask Moscow to mediate the conflict.
Meanwhile, in Tripoli, daily life has become more onerous. One woman said Wednesday that she had waited more than three days on line to get a half-tank of fuel for her car.
The woman said that, while waiting in the line on Tuesday night, she saw a man a few cars ahead of her reach the fuel pump and present his number to a police officer, who rejected it as fraudulent. When the man protested, the police officer fatally shot him, she said.
Witnesses torched a police car in retaliation, she said.
Life outside the capital also appears precarious. The same woman said about three dozen Gadhafi soldiers on Saturday broke into her family's farm near Zwara -- just east of the Tunisian border -- and moved in.
One of the soldiers told the farm caretaker they were taking over civilian properties to use as command posts and weapons storehouses because they feared being attacked by NATO if they were to use military facilities, said the woman, who asked not to be identified because she fears reprisal.
A number of residents of the city of Yefren, about two hours southwest of Tripoli, had taken up residence in caves in order to avoid pro-Gadhafi forces, said an elderly man who cited safety concerns in declining to be identified.
Nearby residents avoid seeking help from Yefren's hospital, he said. "Any injured person who tries to enter the hospital is detained by the brigades," he said. "We try to treat our wounded and give them first aid and protect them inside the mountain so they won't be killed or kidnapped by Gadhafi's brigades."
CNN's Charley Keyes, Eve Bower, Nima Elbagir and Raja Razek contributed to this report.