Bangladesh's prime minister says new polls could be called in the troubled south Asian nation if the opposition ends the violent protests in which hundreds have died in recent months.

Sheikh Hasina Wajed also rejected charges that the boycott by opposition parties of the general election held on Sunday had undermined the legitimacy of her party's landslide victory.

On Monday, US state department spokeswoman Marie Harf expressed US disappointment in the election, saying the results "do not appear to credibly express the will of the Bangladeshi people".

The ruling Awami League, which had already won over a third of the 300 seats in the national parliament before the vote, added 104 to its total on Sunday.

Hasina's government now has a three-quarters majority with a minor allied party playing the role of opposition.

"An election can happen any time [the opposition] comes for a dialogue, but they must stop violence," Hasina told reporters in Dhaka.

However, though senior Awami League officials privately admit that a new poll would have to be held to gain a "resounding mandate", they have also said they believe holding it too soon would be "detrimental to the security and economy of the country".

The main opposition party, the Bangladesh Nationalist party (BNP), boycotted the elections after the Awami League refused to install a neutral caretaker administration before the poll. Government officials say they wanted the BNP to participate, even offering a choice of ministerial posts.

The BNP, led by Khaleda Zia, has alternated in power with the Awami League since democracy was restored to Bangladesh in 1991. Five years later, the Awami League boycotted elections and forced new polls within months. Then in 2007, amid political chaos, the army stepped in.

"There is a sense of deja vu about all this terrible, destructive animosity," said Farzana Shaikh, an expert at the Chatham House thinktank in London.

However, observers have noted the absence of mass protests in recent weeks, with clashes mainly involving small groups of activists from either side.

The BNP say this is because a harsh crackdown has prevented supporters assembling for large demonstrations. Many of the party's senior officials have been arrested. Zia, its leader, has been effectively confined to her Dhaka home for almost a week.

Supporters of the Awami League, and some independent analysts, say that the BNP's protest movement has little support among ordinary people struggling with poor services and rising prices.

Casualties of the violence have included many innocent bystanders, including children, which may also have sapped popular backing for the opposition.

Eighteen people were killed in the violence on Sunday, leading local newspapers to dub the poll the most violent ever in Bangladesh.

Five more deaths were reported on Monday.

The exact turnout in the election – a crucial gauge of its legitimacy - remains contested. Government officials claim that more than 50% of eligible voters participated. The opposition alleges fraud and one estimate, by a consortium of NGOs and unofficial monitors, put the turnout at just over 30%. Election commission officials said the turnout was 41%, local media reported on Monday night.

Relatively robust economic growth has been hit by the political crisis and diplomats in Dhaka have expressed concern that progress in eradicating poverty may lag or even be reversed.

Syeeda Warsi, a British Foreign Office minister, issued a statement on Monday calling the election disappointing.Election commission officials said the turnout was 41%, local media reported late on Monday night.

"The true mark of a mature, functioning democracy is peaceful, credible elections that express the genuine will of the voters. It is therefore disappointing that voters in more than half the constituencies did not have the opportunity to express their will at the ballot box and that turnout in most other constituencies was low," Warsi said.

The European Union, the US and the Commonwealth refused to send observers for Sunday's election.

Though contact is continuing between the major parties, it is unlikely that any deal that would quell street violence will be reached soon.

One obstacle to an agreement will be the series of trials of senior opposition figures, mainly from the country's biggest Islamist organisation Jamaat Islami (JI), who have been accused of war crimes committed during the brutal 1971 civil war that led to Bangladesh's independence from Pakistan. One JI leader, Abdul Quader Mollah, was hanged last month and more may follow him to the gallows.

Hasina told reporters on Monday that the trials would continue and that sentences would be handed down.

The majority of the casualties at the weekend were identified as activists from opposition groups – including JI – who clashed with security forces around the country. There were more than 100 arson attacks on poll centres.

"The new factor now is the Jamaat [Islami] and we are seeing a shift in the political landscape, the outcome of which is not quite clear at the moment. This is as much about a struggle for the identity of the state as about zero-sum politics," said Shaikh, the Chatham House expert.

JI was effectively banned from participating in elections by a court last year and has spearheaded many of the opposition protests in recent months.