June 13 (Reuters) - Burundi's budget will rise 15.9% to 4.4 trillion Burundi francs ($1.5 billion) in the 2024/25 financial year starting in July, largely because of the unfreezing of civil servants' salary bonuses, finance minister Audace Niyonzima said on Thursday.

The landlocked country of 13.2 million, where 80% of the population is employed in the agricultural sector, is one of the poorest and most densely populated in the world.

The economy is expected to grow 5.4%, up from 4.2% in 2023/24, boosted by government investment programmes and improved cooperation with the country's development partners, Niyonzima said.

The economy's rosier outlook is underpinned by favourable rainfall, a gradual resumption of investment in the mining sector, strategic public investment, and the knock-on effects of fiscal, monetary, and foreign exchange policy reforms, the World Bank says.

Salaries were frozen in 2015 by the then government after donors slapped sanctions on the east African country following political turmoil. The sanctions were lifted in 2021.

The budget was expected to be passed by lawmakers later on Thursday.

"The priority allocations retained in this draft budget, which are at the origin of the budget increase, include particularly salaries and bonuses which have increased significantly," Niyonzima said.

The budget would also channel funds towards the construction of road infrastructure in rural and urban areas, he said.

The budget will be funded equally from internal and external revenues, leaving a forecast deficit of 441.9 billion francs next financial year, up from 426.5 billion francs in 2023/24, he said.

($1 = 2,858.8400 Burundi francs) (Reporting by Hereward Holland Editing by Mark Potter)