Syrian President Bashar al-Assad has said that he intends to retake "the whole country" from rebel forces.
In a rare interview, he told AFP news agency that defeating the groups ranged against him could take some time due to the involvement of regional powers.
World powers have agreed to push for a cessation of hostilities in a week's time.
The UN says it hopes to start delivering aid to some besieged areas in Syria within the next 24 hours.
Mr Assad expressed support for peace talks but said negotiations did not mean "we stop fighting terrorism".
US State Department spokesman Mark Toner said Mr Assad was "deluded" if he thought there was a military solution to the conflict.
More than 250,000 people have been killed and some 11 million displaced in almost five years of fighting in Syria.
Some Syrian cities have been cut off from humanitarian aid for more than a year because of fighting. About 13.5 million people are in need of aid, the UN says.
Mr Assad was speaking in Damascus on Thursday, ahead of the deal on the cessation of hostilities that was agreed in Munich late on Thursday night.
In a wide-ranging interview, President Assad spoke about:
- The government offensive on Aleppo: He said it was to sever the rebels' supply route from Turkey
- UN accusations of war crimes: He said recent claims against his government were "politicised"
- Syrians seeking asylum in Europe: Mr Assad blamed Europe's support for "terrorists" for the crisis
He said government forces would try to retake all of Syria "without any hesitation", but that the involvement of regional powers meant that "the solution will take a long time and will incur a heavy price".
Syrian government forces, backed by Russian air strikes, have almost encircled rebels in parts of the northern city of Aleppo and cut off their main supply route from the Turkish border.
Mr Assad told AFP that if all rebel supply routes from Turkey, Jordan and Iraq were cut, the country's "problem" could be solved in less than a year.
Mr Assad rejected recent accusations by the UN that his government was guilty of war crimes.
Earlier this month, UN human rights investigators said the Syrian government had carried out a state policy of extermination against thousands of detainees.
In a report for the UN Human Rights Council, they accused the Syrian government of crimes against humanity. The report also said government and rebel forces had committed possible war crimes.
But Mr Assad called the accusations "politicised" and said the investigators had provided no evidence.
Tentative peace talks were held in Geneva earlier this month and have been "paused" until 25 February.
Mr Assad said he "fully believed in negotiations and in political action since the beginning of the crisis."
"However, if we negotiate, it does not mean that we stop fighting terrorism. The two tracks are inevitable in Syria," he said.
He also told AFP he believed there was a risk that Saudi Arabia and Turkey, which both back Syrian rebel forces, would intervene militarily in Syria.
Tens of thousands of people have fled the fighting around Aleppo, raising fears of a humanitarian crisis.
Many have headed to the Turkish border but have not been allowed to cross. Instead, Turkey says it will support the refugees on the Syrian side of the border.
Both Turkey and the European Union fear those displaced could add to the hundreds of thousands making the often perilous journey to Europe to seek asylum.
Mr Assad said European governments had caused the migration crisis "by giving cover to terrorists in the beginning and through sanctions imposed on Syria".
"I would like to ask every person who left Syria to come back," he said. "They would ask 'why should I come back? Has terrorism stopped?'."
Meanwhile, a new UN task force set up to co-ordinate aid distributions is expected to convene in Geneva later on Friday.
"The UN system has been geared to deliver this aid all along, especially to besieged areas, and that's precisely what's going to be discussed today: how to start, and when to start," UN spokesman Ahmad Fawzi said.
"We hope to start as early as tomorrow, immediately after the meeting, decisions will be taken to roll the aid in, especially to besieged areas that need it", he added.
Riad Hijab, co-ordinator of Syria's main opposition bloc, told the BBC's Newsnight programme that to announce a cessation of hostilities before making progress in the political process "is not realistic, objective or logical".
"It only satisfies the Russian demand to preserve gains by Russian and Iranian forces through their scorched-earth policy against rebels in territories under their control," he said.
Syria conflict - key questions
Why is there a war in Syria?
Anti-government protests developed into a civil war that, four years on, has ground to a stalemate, with the Assad government, Islamic State, an array of Syrian rebels and Kurdish fighters all holding territory.
Who is fighting whom?
Government forces concentrated in Damascus and the centre and west of Syria are fighting the jihadists of Islamic State and al-Nusra Front, as well as less numerous so-called "moderate" rebel groups, who are strongest in the north and east. These groups are also battling each other.
How has the world reacted?
Iran, Russia and Lebanon's Hezbollah movement are propping up the Alawite-led Assad government, while Turkey, Saudi Arabia and Qatar back the more moderate Sunni-dominated opposition, along with the US, UK and France. Hezbollah and Iran are believed to have troops and officers on the ground, while a Western-led coalition and Russia are carrying out air strikes.