Leaders from the world's strongest economies have gathered in the Australian city of Brisbane, commencing the G20 summit, with the crisis in Ukraine expected to take centre-stage.

The two-day summit, which began on Saturday, promises to be a showdown between Western leaders and Russian President Vladimir Putin, amid fresh reports of Russian troops pouring into eastern Ukraine.

Al Jazeera's Scot Heidler, reporting from Brisbane, said: "We thought that it [Russia-Ukraine issue] would hijack the discussions. It definitely distracted the initial couple of hours of the summit, but it hasn't really hijacked it."

At a news conference in Canberra on Friday, British Prime Minister David Cameron blasted Russia's actions as "unacceptable", warning they could draw greater sanctions from the United States and the European Union.

It is a large state bullying a smaller state in Europe, and we have seen the consequences of that in the past.

David Cameron, British Prime Minister

"It is a large state bullying a smaller state in Europe, and we have seen the consequences of that in the past," Cameron said.

Russia denies sending troops and tanks into Ukraine, but increasing violence, truce violations and reports of unmarked armed convoys travelling from the direction of the Russian border have aroused fears that a shaky September 5 truce could collapse.

 As the talks were held, official sources reported that at least five civilians, including two children, were killed in fighting in eastern Ukraine. The deaths occured in Gorlivka, near the rebel stronghold of Donetsk, late on Friday.

The local council of the city added that 12 people were also injured when an artillery struck a residential area of the city, seriously damaging two apartment blocks and destroying six flats. It did not specify who fired the mortars.

Security issues

The G20 leaders summit in Brisbane is focused on boosting world growth, fireproofing the global banking system and closing tax loopholes for giant multinationals.

But with much of the economic agenda agreed and a climate-change deal signed last week in Beijing between the United States and China, security concerns are moving to the forefront.

In addition to Ukraine, the crises in the Middle East are threatening to overshadow the economic agenda. British nationals who become foreign fighters abroad could be prevented from returning home under new laws to deal with fighters in conflicts such as Iraq and Syria, Cameron said in an address to the Australian parliament on Friday.

As host, Australia will continue pushing its growth agenda, despite growing security tensions, Prime Minister Tony Abbott said at the joint news conference with Cameron.

Australia is pushing for an increase in global growth targets of two percent by 2018 to create millions of jobs, and that goal appears on track. Over 1,000 policy initiatives proposed by G20 nations should add around 2.1 percent, the head of the Paris-based Organisation for Economic Co-operation and Development said.

The G20 is made up of 19 countries - Argentina, Australia, Brazil, Britain, Canada, China, France, Germany, India, Indonesia, Italy, Japan, Republic of Korea, Mexico, Russia, Saudi Arabia, South Africa, Turkey and the United States - and the 28-member European Union.

The group accounts for 80 percent of world trade and 85 percent of global economic production.

Source: Agencies